Cognitive behavior therapy (CBT) is an effective treatment for anxiety and fear-related disorders. However, some patients fail to achieve complete remission or show a return of anxiety symptoms following treatment. The purpose of the study is to determine the extent to which one’s emotional reaction to a CO₂ inhalation task can help predict who might or might not respond to therapy. If CO₂ reactivity proves to be successful in predicting who will benefit from CBT versus who will not, clinicians can make better decisions about which patients should be treated with CBT.