OBJECTIVES: Research has shown the effectiveness of mindfulness-based interventions for a variety of emotional problems in different samples, but it is unknown which factors influence this effectiveness. Therefore, the aim of the current study was: which factors (demographic, personality, and baseline levels of mindfulness skills) moderate the effectiveness of Mindfulness-Based Cognitive Therapy (MBCT)? METHODS: Outpatients with diabetes (type 1 or type 2; N=139) and an elevated level of emotional distress participated in the Diabetes and Mindfulness (DiaMind) trial. They were randomized into MBCT (N=70) or a control group (N=69) that received treatment as usual and that was offered the intervention 6months later. Primary outcomes were anxiety, depressive symptoms, and perceived stress at post-intervention and at 6-month follow-up. RESULTS: Mixed models analyses showed that sex, extraversion, and baseline acting with awareness were significant moderators of effectiveness. In the MBCT group, women showed larger decreases in anxiety and depression across time (large effects) compared to men (medium to small effects). For extraversion divided into quartiles, the three lowest quartiles generally exhibited large decreases in symptoms, whereas the high extraversion group showed medium (perceived stress) to small (depression) decreases. CONCLUSION: MBCT seems to be effective to decrease symptoms of anxiety, depression, and perceived stress for a broad range of person characteristics in patients with diabetes. However, men and those high in extraversion showed considerably lower effectiveness compared to the other groups. The small effect in high extraverts may be due to the large dropout in this subgroup.