Method: A reanalysis of data from Michie, Abraham, Whittington, McAteer, and Gupta (2009) was performed. These data included effect sizes from 122 interventions targeted at physical activity and healthy eating, and the coding of the interventions into 26 BCTs. A CART analysis was performed using the BCTs as predictors and treatment success (i.e., effect size) as outcome. A subgroup meta-analysis using a mixed effects model was performed to compare the treatment effect in the subgroups found by CART. Results: Meta-CART identified the following most effective combinations: Provide information about behavior–health link with Prompt intention formation (mean effect size g ϭ 0.46), and Provide information about behavior–health link with Provide information on consequences and Use of follow-up prompts (g ϭ 0.44). Least effective interventions were those using Provide feedback on performance without using Provide instruction (g ϭ 0.05). Conclusions: Specific combinations of BCTs increase the likelihood of achieving change in health behavior, whereas other combinations decrease this likelihood. Meta-CART successfully identified these combinations and thus provides a viable methodology in the context of meta-analysis. Keywords: intervention effectiveness, behavior change techniques, synergistic effects, classification and regression trees, subgroup, meta-analysis