BACKGROUND:Mindfulness-based interventions for depression and anxiety emphasize the importance of observing present moment experience, but observing has often been positively related to anxiety and unrelated to depression symptoms. The current study sought to better understand the conditions and mechanism through which observing relates to symptoms by examining six conditional process models in which (1) nonreactivity moderates the direct effect of observing on symptoms of anxiety and depression symptoms and (2) nonreactivity moderates the indirect effect of observing on anxiety and depression via cognitive emotion regulation strategies (i.e., rumination, worry, and reappraisal). METHODS: A clinical sample of 189 adults with anxiety and depressive disorders completed the Five Facet Mindfulness Questionnaire, Mood and Anxiety Symptom Questionnaire, Penn State Worry Questionnaire, Ruminative Responses Scale, and Emotion Regulation Questionnaire. RESULTS: Conditional process models showed that nonreactivity significantly moderated the direct effect of observing on symptoms of depression, but not anxiety. Additionally, nonreactivity significantly moderated the indirect effect of observing on symptoms of depression through rumination and reappraisal, but not worry. For anxiety, nonreactivity significantly moderated the indirect effect of observing on symptoms through worry and rumination, but not reappraisal. LIMITATIONS: Causal interpretations of results are limited. CONCLUSION: Findings suggest that the relationship between observing and symptoms of depression and anxiety depends on the capacity to observe nonreactively, which may influence symptoms directly and indirectly through cognitive emotion regulation strategies. Findings raise important implications for tailoring mindfulness-based treatments for anxiety and depression symptoms.