Our understanding of how Behavioral Activation (BA) for depression works is limited. BA is theorized to lead to changes in depression through changes in activation. While distal support for activation as a mechanism has been obtained, more research is needed before definitive conclusions can be drawn. Research on mechanism should consider the appropriate time-frame for examining changes in the theorized mechanism variable and whether the proposed mechanism is expected to exert causal influence in all BA cases. These issues were considered in the current study in which a post-hoc analysis was conducted to explore BA’s mechanism using single-subject data obtained at each session during the course of treatment.