The dual-factor model of mental health suggests that enhancing positive mental health and alleviating psychopathology do not automatically go hand-in-hand. This study investigates the relationship between the effectiveness on depression/anxiety symptoms and positive mental health of Acceptance and Commitment Therapy (ACT). It draws on RCT data (n = 250) of a self-help ACT. Patients’ depression/anxiety symptoms and positive mental health were completed at baseline, at post-intervention after nine weeks, and at follow-up after five months. Percentage of unique variance of depression/anxiety symptoms explained by positive mental health (and vice versa), and the degree of classificatory agreement between improvements in positive mental health and depression/anxiety, were examined using regression analysis and Reliable Change Index (RCI). Positive mental health, i.e. baseline and change, explained 15% and 12% of the variance in follow-up depression and anxiety symptoms, beyond the 7% and 9% that was explained by baseline levels of depression and anxiety. Depression and anxiety symptoms, i.e., baseline and change, explained 10% and 9% of the variance in follow-up positive mental health, on top of the 35% that was explained by baseline levels of positive mental health. Cross-classification of the Reliable Changes showed that 64% of the participants that improved during the ACT-intervention, improved on either depression symptoms or positive mental health, and 72% of the participants improved on either anxiety symptoms or positive mental health. The findings support the dual-factor model and suggest that it is important to systematically implement measures of both psychopathology and positive mental health in mental health care and therapy evaluations.