This article demonstrates a novel application of propensity score matching techniques: to estimate nonexperimental impacts on program participants within the context of an experimental research design. The author examines the relationship between program participation, defined as qualifying for an earnings supplement by working full-time, and marital union formation among low-income mothers in two Canadian provinces. The author finds that receipt of an earnings supplement substantially increased union formation in one province but not the other. A subgroup analysis based on propensities of program participation revealed that the positive effect on unions was concentrated among relatively disadvantaged participants. The techniques demonstrated in the article are broadly applicable to studies in which take-up is less than 100% among those randomly assigned to a program group. (Contains 4 figures, 3 tables, and 8 notes.)