Obese children were randomly assigned to a family-based behavioral treatment that included either stimulus control or reinforcement to reduce sedentary behaviors. Significant and equivalent decreases in sedentary behavior and high energy density foods, increases in physical activity and fruits and vegetables, and decreases in standardized body mass index (z-BMI) were observed. Children who substituted active for sedentary behaviors had significantly greater z-BMI changes at 6 (−1.21 vs. −0.76) and 12 (−1.05 vs. −0.51) months, respectively. Substitution of physically active for sedentary behaviors and changes in activity level predicted 6- and 12-month z-BMI changes. Results suggest stimulus control and reinforcing reduced sedentary behaviors are equivalent ways to decrease sedentary behaviors, and behavioral economic relationships in eating and activity may mediate the effects of treatment.