Sensitivity to differences in reinforcement contingencies was examined in 6 individuals with mental retardation. A concurrent operants procedure was implemented in the context of a computer game. Participants selected two different types of animated figures displayed concurrently in the left and right portions of a touchscreen monitor. Over a series of conditions, the relative rates or magnitudes of reinforcers following selections of the two options were changed in ratios of 5:1, 3:1, 1:1, 1:3, and 1:5. Sensitivity was quantified by application of the generalized matching equation. Results included individual differences in sensitivity and differential sensitivity to rate and magnitude variation. The results suggest that comprehensive assessments of potential reinforcers may benefit from including tests of delivery parameters.