Initial research suggests that acceptance and commitment therapy (ACT) is efficacious in treating hoarding problems, but processes of change in ACT for hoarding are unclear. Thus, we investigated potential mediators and moderators in a randomized waitlist-controlled trial (n = 73) of web-based ACT for U.S. adults with clinically significant hoarding problems. Hoarding severity was significantly correlated with hoarding-related psychological inflexibility (r = 0.52, p < .01), mindful awareness (r = −0.40, p < .01), and beliefs that people with hoarding disorder are different from others (r = 0.31, p < .01). Effects of ACT were not significantly moderated by age, gender, or baseline hoarding-related psychological inflexibility. Longitudinal mediation analyses did not support mediation by hypothesized processes. However, the effect of condition on hoarding severity at follow-up was significantly mediated by hoarding-related psychological inflexibility at follow-up (ab = −0.41 [95% CI: 0.85, −0.01]), and change in hoarding-related psychological inflexibility (r = 0.73, p < .01) and mindful awareness (r = −0.69, p < .01) was correlated with change in hoarding symptoms over time. Overall, our results suggest that psychological inflexibility, self-stigma, and mindful awareness are indeed related to hoarding symptoms, but we cannot conclude that they underly the effects of ACT for hoarding. Limitations include a predominantly White and female sample.