<p><b>Background</b> Predicting response to cognitive behavior therapy (CBT) assists efforts to enhance treatment outcome when predictive factors are modifiable prior to, or during, treatment. The extent to which clients hold beliefs and attitudes consistent with CBT (termed CBT-mindedness) is a relatively new concept with research suggesting it predicts response to CBT amongst small samples of adults with anxiety. This study aimed to investigate CBT-mindedness amongst a larger clinical population receiving internet-delivered CBT (iCBT).</p><p><b>Method</b> 1132 adults with anxiety, depression or mixed anxiety and depression who accessed iCBT with or without therapist support via the THIS WAY UP clinic completed a brief self-report measure of CBT-mindedness along with measures of distress, anxiety, and depression. Measures were completed pre- and post-treatment.</p><p><b>Results</b> The 3-factor structure of the CBT Suitability Scale (CBT-SUITS) was confirmed and scores were unrelated or very weakly related to symptoms/distress. CBT-mindedness increased amongst treatment completers. CBT-mindedness predicted post-treatment distress (but not symptoms), and change in CBT-mindedness predicted lower post-treatment symptoms and distress.</p><p><b>Conclusions</b> The CBT-SUITS represents a psychometrically sound measure of CBT-mindedness. Results amongst this large sample of adults accessing iCBT in a community service indicate that CBT-mindedness (or CBT-mindedness change) is an important predictor of therapy response.</p>