Background: Many online interventions designed to promote health behaviors combine multiple behavior change techniques (BCTs) and additional modes of delivery (MoD, e.g. text messages) to maximize effectiveness. Also, usability factors may influence effectiveness. This study aims to identify synergistic effects of BCTs, MoDs and usability factors. Methods: A re-analysis of data from Webb (2010) was performed. Eighty interventions ((g ) ¯ = 0.16) were included, targeting a variety of health behaviors. 26 BCTS and 11 MoDs were coded in the interventions, and a usability taxonomy was developed. Meta-CART (Classification and Regression Trees) analyses were performed using the BCTs and MoDs as predictors, and effect size as outcome. Findings: Meta-CART identified one synergistic effect: Interventions that included barrier identification/ problem solving and prompted participants to plan social support/ social change but did not provide normative information about the behavior of others had an average effect size of 0.25 (95%CI=0.08-0.42). No synergistic effects were found for MoDs, or MoDs combined with BCTs. Discussion: The present analyses provide guidance on developing effective online interventions for promoting health behavior change. Only one combination was effective, indicating most BCTs did not have synergistic effects above their univariate effect found by Webb et al.