Reinforcing value of a behavior refers to the motivation to engage in the behavior. A reinforcing behavior will support more work to obtain the behavior. Individual differences in the reinforcing value of physical activity predict the usual physical activity in children. Another factor that may influence physical activity is liking of physical activity. Liking or hedonics refers to an affective rating associated with the behavior, and people are more likely to engage in physical activities that they like than ones that they do not like. Liking correlates with physical activity in youth. Although the independence of reinforcing value and liking of physical activity has not yet been tested, the motivation to gain access to a behavior and liking for that behavior are likely different constructs. PURPOSE: To determine whether liking and relative reinforcing value (RRV) of physical activity independently predict time youth spend in moderate-to-vigorous physical activity (MVPA). METHODS: Boys (n = 21) and girls (n = 15) age 8 to 12 years were measured for height, weight, aerobic fitness, liking and RRV of physical activity, and minutes in MVPA using accelerometers. RESULTS: Using multiple regression to control for individual differences in age, sex, BMI percentile, aerobic fitness, and time the accelerometer was worn, liking (P < 0.05) and RRV (P < 0.01) of physical activity independently predicted time in MVPA. When using median splits of the RRV and liking data to form subject groups, the group of children with both a high liking and RRV of physical activity participated in greater (P < 0.05) minutes per week of MVPA (1340 + 72 min) than groups with high RRV-low liking (1040 + 95 min), low RRV-high liking (978 + 89 min), or low RRV-low liking (1007 + 70 min) of physical activity. CONCLUSIONS: RRV and liking of MVPA are separate constructs as they independently predict MVPA of children. Those children who find physical activity the most reinforcing and also have a high liking of physical activity engage in 33% more MVPA than children who either find physical activity highly reinforcing or have a high liking of physical activity. Interventions that concurrently increase the reinforcing value and liking of physical activity may be the most effective for increasing youth participation in free-living MVPA. Supported by NIH Grant RO1 HD42766.