Validity and Reliability of the Social Interaction Anxiety Scale (SIAS) and the Social Phobia Scale (SPS) Background: The Social Interaction Anxiety Scale (SIAS) and the Social Phobia Scale (SPS) are commonly used self-report instruments for social anxiety. Disorder-specific measures for social phobia are urgently needed considering the high prevalence of this anxiety disorder. The present study reports validity and reliability properties of the German version of both scales. Methods: 76 participants diagnosed with social phobia, 199 participants with panic disorder with/without agoraphobia or agoraphobia without a history of panic disorder, and 82 participants with other clinical disorders were included in the analyses. The patients were seeking treatment in the Christoph-Dornier Foundation for Clinical Psychology. The assessment procedure at the Foundation comprised a structured clinical interview and a battery of questionnaires. SPS und SIAS were part of this battery. Results: The results indicate that the scales have good internal consistency, adequate convergent, but inadequate discriminant validity with similar/different measures of anxiety and mood disturbances and they sensitively measure treatment change. Both scales discriminated between the clinical samples. However, the classification performance decreased significantly when all three samples were considered. A factor analysis did not support the originally theoretically derived uniformity of the scales. Conclusions: These findings confirm the utility of the questionnaires only in part. It is recommended to complete the assessment of social anxiety with other questionnaires like the Social Phobia and Anxiety Inventory (SPAI).