Given the prevalence of depression, it is worthwhile to consider a variety of treatment approaches to reach as many sufferers as possible, including highly accessible formats such as self-help books. Books based in acceptance and commitment therapy (ACT) and cognitive behavioral therapy (CBT) propose to treat depression through distinct processes of change, though the degree to which these treatments are distinguishable in this format is unclear. Furthermore, it is possible that some individuals may respond better to therapeutic processes from one approach over the other based on personal preferences. We tested the effects of ACT and CBT self-help books on processes of change in a sample of 139 depressed college students in which some participants were given a choice of treatment and others were randomized. Cognitive fusion, which improved better in the ACT group, was the only process of change that distinguished the two treatments. Additionally, early improvements in cognitive fusion were associated with less depression-related stigma at posttreatment. Lastly, randomization, instead of choosing a treatment, led to greater improvements in almost all processes of change. We discuss how these findings inform personalized care, tangible differences between ACT and CBT, and effective practices for treating depression at large scale.