750 ILCS 5/510: Modifying Child Support and Ending Maintenance

Under Illinois law, section 5/510 of the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5/510) lets you modify or terminate an existing maintenance or child support order, but only if you prove a substantial change in circumstances since the last order, and any modification applies only to payments that come due after you file and serve notice of your motion.¹ Past-due amounts are locked in. Maintenance also ends automatically on death, remarriage, or proven cohabitation, without any motion at all.

What Counts as a Substantial Change

Both maintenance and child support can only be modified after you show a substantial change in circumstances since the last order was entered.¹ Courts treat this as a real threshold. Wanting to pay less or receive more is not enough. The person filing carries the full burden of proving that the current arrangement has become unfair because of new, significant developments: a major income change, a serious health event, a job loss beyond your control, retirement in good faith.

One rule in the statute catches people off guard. Foreseeability is not a defense. If your former spouse argues that the change you point to was predictable when the original order was entered, that argument fails unless the original order or agreement specifically identified that event as something both sides already accounted for.¹ Being near retirement age at the time of divorce, for instance, does not by itself block a later modification when you actually retire.

Check First Whether Your Order Is Modifiable

Before you spend time and filing fees, read your judgment. Illinois law allows divorcing spouses to agree in writing that maintenance cannot be changed in amount, duration, or both.² If that non-modifiability language appears in your judgment, no court can change those terms regardless of how dramatic your circumstances are. A motion filed against a non-modifiable clause gets denied on procedural grounds before anyone reaches the merits. If the agreement is silent on the point, maintenance stays modifiable on the usual substantial-change showing.

Factors the Court Weighs for Maintenance

Once you clear the threshold, the court works through nine factors in section 510(a-5) together with the broader factors in section 504(a).¹ They fall into recognizable categories:

Any change in either party’s employment status, and whether the change was made in good faith. Quitting a high-paying job to reduce your obligation will not help.
Efforts the recipient has made toward becoming self-sufficient.
Impairment of either party’s earning capacity, for example through disability, illness, or time out of the workforce.
Tax consequences of maintenance for each party.
How long maintenance has been paid compared to the length of the marriage.
What property each party received in the divorce, including retirement benefits, and its current value.
Increases or decreases in each party’s income since the last order.
Property either party has acquired since the judgment.
Any other factor the court finds relevant and equitable.

A recipient who has landed a good job, finished a degree, or built a business may see maintenance reduced or ended. An unexpected health crisis or involuntary job loss on either side can support an increase or extension. If your maintenance runs for a fixed term and you want it extended, file before the end date. Once a fixed-term order expires, the obligation is simply gone.

Modifying Child Support

Child support uses the same substantial-change standard, then adds a recalculation.¹ The court runs the numbers again under the Income Shares model in section 505, which combines both parents’ monthly net income and applies a schedule based on the number of children to arrive at the basic support obligation.³ Each parent’s share is proportional to their percentage of the combined income.

Typical grounds include a significant income change for either parent, a shift in parenting time that changes the cost of caring for the child, or changes in the child’s health or educational needs. A jump in the child’s health insurance premium can matter too, because that cost feeds into the calculation. Courts can deviate from the guideline number when applying it would not serve the child’s best interests, but any deviation requires specific written findings.

Imputed Income for Voluntarily Unemployed Parents

A parent cannot dodge support by choosing not to work or by taking a deliberately lower-paying job. When a court finds voluntary unemployment or underemployment, it calculates support based on what the parent could be earning, an approach called imputed income.³

The court looks at work history, education, job skills, age, health, criminal record, efforts to find work, local job market conditions, and prevailing wages in the community. When there isn’t enough history to estimate earning capacity, the court presumes potential income at 75% of the federal poverty guidelines for a single person. Incarceration does not count as voluntary unemployment.³ Income can only be imputed after an evidentiary hearing or by agreement, and the judge must issue specific written findings. If you are facing imputation, document job applications, interview records, and any medical issues limiting your ability to work.

File Immediately: No Retroactive Relief

This is the timing rule that matters most. A modification can only affect payments that come due after you give proper notice of your motion.¹ Every payment that came due before you filed is locked in as a judgment and cannot be reduced or forgiven by any court in Illinois or in any other state.⁴

If your income drops or your circumstances change, file now. Every month you wait is another month of the old amount becoming an enforceable arrearage. Six months of delay after a job loss produces six months of arrears at the old rate with no way to undo them. Federal law ties the court’s hands.

When Maintenance Ends Automatically

Section 510(c) ends maintenance in three situations, without anyone filing a motion, unless the parties agreed otherwise in writing:¹

Either spouse dies.
The recipient remarries. The obligation ends by operation of law on the date of the remarriage.
The recipient begins living with another person in an ongoing, marriage-like relationship. The payor petitions the court, and if cohabitation is proven, maintenance ends as of the date the court finds it began.

What Counts as Cohabitation

Cohabitation generates most of the fights. The statute uses the phrase “resident, continuing conjugal basis,” which effectively means a relationship that looks like a marriage. Courts examine the totality of the circumstances rather than a checklist. Common considerations include how long the relationship has lasted, how much time the couple spends together, whether they share finances, whether they vacation and spend holidays together, and how far their personal affairs are intertwined. Casual dating, even with occasional overnight stays, does not clear the bar. The court is looking for signs of permanence and mutual dependence.

When Child Support Ends

For support purposes, a “child” under Illinois law means anyone under 18, plus anyone between 18 and 19 who is still attending high school.³ Support does not automatically stop on the 18th birthday if the child is still in high school; it runs until graduation or age 19, whichever comes first. Section 5/510 doesn’t govern contributions to college costs, which sit in a separate provision (section 513) with their own rules and their own age limits.

How to File a Modification

You’ll prepare two core documents: a motion to modify the existing order and a financial affidavit detailing your current income and expenses. Bring supporting proof of both your finances and the change you rely on. Recent pay stubs, your most recent tax returns, and evidence of the triggering event: a termination letter after a job loss, medical records after a health change.

Illinois requires nearly all court filings to go through the statewide electronic filing system, eFileIL, which accepts submissions from attorneys and self-represented filers around the clock.⁵ If you qualify for an e-filing exemption, you can file in person at the circuit clerk’s office. Filing fees vary by county. If you cannot afford the fee, apply for a fee waiver using the standardized forms on the Illinois Courts website.⁶

After filing, serve the other party with notice of your motion. Service is legally required; the court cannot hear the motion until the other side has been properly notified. Use a process server, the sheriff’s office, or another method the rules allow. Once service is on file, the court will set a hearing where both sides present financial evidence. Because modification cannot reach back before your filing and service date, that date effectively sets the starting line for any change the court orders. Delay costs real money.

1
Illinois General Assembly. Illinois Code 750 ILCS 5/510 – Modification and Termination of Provisions for Maintenance, Support, Educational Expenses, and Property Disposition
2
Illinois General Assembly. Illinois Code 750 ILCS 5/502 – Agreements
3
Illinois General Assembly. Illinois Code 750 ILCS 5/505 – Child Support; Contempt; Penalties
4
Office of the Law Revision Counsel. 42 USC 666 – Requirement of Statutorily Prescribed Procedures
5
State of Illinois Office of the Illinois Courts. eFileIL – Statewide eFiling
6
Illinois Courts. Approved Statewide Forms – Fee Waiver for Civil Cases