The present study compares the impact of individualized treatment provided by trainee therapists based on a traditional cognitive behavior therapy (CBT) and acceptance and commitment therapy (ACT) model. Fourteen therapists were given initial training in CBT and ACT. Outpatients (N = 28) were randomized to either approach, with each therapist treating one client within each model, linked to a functional analysis. Clients treated within an ACT model showed better symptom improvement than the CBT clients, despite the fact that students felt initially less knowledgeable about ACT and were more fearful throughout when it was used. CBT improved client self-confidence more rapidly than ACT, and ACT improved acceptance more than CBT. Both processes predicted better outcomes; acceptance remained predictive when controlling for self-confidence but not vice versa. Overall, therapists with limited training in both models got better results with ACT and the processes of change fit with the ACT model. (Contains 5 tables and 1 figure.)