Randomized clinical trials (e.g., Teasdale, Segal, Williams, Ridgeway, Soulsby, & Lau, 2000; Ma & Teasdale, 2004) support the efficacy of Mindfulness-Based Cognitive Therapy for preventing depressive relapse. Unfortunately, there are scant studies of the treatment’s mechanisms of action. Teasdale and colleagues (2000) have speculated that MBCT is effective because it enhances participants’ ability to disengage from ruminative thinking by relating to their thoughts as mental events, rather than accepting them as reality (i.e., decentering). The current research sought to test the hypothesis that MBCT treatment enhances participants’ ability to prevent and/or disengage from, mood-reactive depressogenic thinking and leads to an increase in self-reported mindfulness and decentering. Individuals with a history of recurrent major depressive disorder, who were not currently depressed, completed the Five-Facet Mindfulness Questionnaire and the Experiences Questionnaire. They also participated in three objective tasks incorporating sad mood inductions, psychophysiological measures, a think aloud period, and the sustained attention to response task. These tasks were designed to assess participants’ ability to prevent and/or disengage from, mood-reactive depressogenic thinking. Participants were then randomized to MBCT or a waitlist control group. Approximately two months later, participants were re-assessed for mindfulness, decentering, and changes in their ability to prevent and/or disengage from depressogenic thinking in response to the sad mood inductions.