Background: Assessment of psychiatric disorders in persons with an intellectual developmental disorder (IDD) can be performed with a variety of greatly differing instruments. This makes the choice of an instrument best suited for the intended purpose challenging. In this study, we developed a comprehensive set of characteristics for the evaluation and description of assessment instruments for psychiatric disorders in adult persons with IDD. This simplifies the search for an instrument as it makes an easy and direct comparison possible and hereby allows a more thorough and appropriate decision making when selecting assessment tools. Method: A mixed-methods approach was used. First, a systematic literature search was conducted to identify existing tools for the description and evaluation of assessment instruments. Second, the content of these tools was combined and missing features and IDD-specific attributes were added. Finally, expert consultations were performed. Results: The Characteristics of Assessment Instruments for Psychiatric Disorders in Persons with Intellectual Developmental Disorders (CAPs-IDD) lists characteristics to evaluate and describe instruments for psychiatric disorders in persons with IDD. It comprises two sections: first, the conceptual and measurement model; second, the psychometric properties. Each section consists of various subsections and a detailed response format for coding instruments. Conclusions: The use of the CAPs-IDD helps to identify and choose instruments that best suit the respective purpose. Filled with information, it could be made accessible via new technologies to researchers and practitioners and be updated when new information is available. Thus, it contributes to a more reliable and valid assessment of possible psychiatric disorders in persons with IDD. (Contains 1 figure and 2 tables.)