The objectives were to examine the relationships among clients' affect regulation capacities, in-session emotional processing, outcome, and the working alliance in 66 clients who received either cognitive-behavioral therapy or process-experiential emotion-focused therapy for depression. Clients' initial level of affect regulation predicted their level of emotional processing during early and working phases of therapy. Clients' peak emotional processing in the working phase of therapy mediated the relationship between their initial level of affect regulation and their level of affect regulation at the end of therapy; and clients' level of affect regulation at the end of therapy mediated the relationship between their peak level of emotional processing in the working phase of therapy and outcome. Clients' affect regulation at the end of therapy predicted outcome independently of the working alliance. The findings suggest that clients' level of affect regulation early in therapy has a significant impact on the quality of their in-session processing and outcome in short-term therapy. Limitations of the study and future directions for research are discussed.