We tested the hypothesis that the tripartite model [Clark, L. A., & Watson, D. (1991). Tripartite model of anxiety and depression: Psychometric evidence and psychometric implications. Journal of Abnormal Psychology, 100, 316-336] can be extended to account for change during treatment for anxiety and depression. Forty-one patients treated naturalistically in private practice with cognitive behavior therapy completed weekly measures of depression, anxiety, negative affect (NA), positive affect (PA), and anxious arousal (AA). Consistent with the model, NA was associated with anxiety and depression during treatment, PA was more strongly related to depression than to anxiety, and AA was more strongly related to anxiety than to depression. As predicted, symptoms of depression and anxiety and NA all decreased during treatment. As predicted, AA also decreased, particularly for patients with panic disorder. PA increased during treatment, but only for patients who showed a significant decline in depression and only over an extended period of treatment. Nearly two-thirds of the variance in anxiety change was accounted for by changes in depression and NA, and just over three-fourths of the variance in depression change was accounted for by changes in anxiety and NA, indicating that much of the change in anxiety and depression across the course of treatment is shared in common.