Six adults diagnosed with obsessive compulsive disorder (OCD) were treated with either acceptance and commitment therapy (ACT), cognitive therapy (CT), or exposure with ritual prevention (ERP) in a preliminary attempt to clarify the similarities or differences between the purported mechanisms of change that underlie these treatments. A new process measure was constructed with items assessing psychological flexibility, cognitive reappraisal, and extinction. This process measure was given weekly along with a measure of OCD severity. Visual analyses suggest that one of two participants in the ACT condition exhibited the highest overall changes on psychological flexibility, while the other participant showed equivalent overall scores on all processes. Both CT participants had highest scores on extinction and psychological flexibility, followed by cognitive reappraisal. ERP had the most consistent results, with both participants generally reporting extinction to be the most notable process of change. Although there was individual variability, raw scores indicate that extinction was the most central mechanism, but that psychological flexibility showed the greatest change when accounting for pretreatment levels of familiarity. Strengths, limitations, and future directions are discussed. (Contains 2 tables, 3 figures, and 1 footnote.)