This paper examines how income levels affected the substitution of public health insurance for private health coverage under expansions of Illinois' State Children's Health Insurance Program (SCHIP). Building on a technique developed by Abadie and Gardeazabal (2003), I estimate that among children whose family incomes are between 200% and 300% of the federal poverty level (FPL), 35% of those covered by SCHIP would have retained private coverage in the absence of SCHIP. Significant substitution also appears between 300% and 400% FPL, but surprisingly I find evidence that the introduction of SCHIP caused an increase in private health insurance coverage for those with family incomes between 400% and 500% FPL.