Background: Interpersonal staff behaviour is one of the instigating factors associated with challenging behaviour in clients with intellectual disabilities (ID). There are several studies focusing on the influence of intrapersonal staff characteristics--such as beliefs, attributions and emotional reactions--on staff behaviour. Little is known, however, about interpersonal staff behaviour itself. This study describes the development and validation of the Staff-Client Interactive Behaviour Inventory (SCIBI), measuring both intrapersonal and interpersonal staff behaviour in response to challenging behaviour in clients with ID. Method: A total of 292 staff members, employed in residential and community services, completed the SCIBI for 34 clients with ID and challenging behaviour. Results: Confirmatory factor analysis of a seven-factor model--with assertive control, hostile, friendly and support-seeking interpersonal behaviour; proactive thinking; self-reflection; and critical expressed emotion as reliable factors--showed an exact fit to the data, indicating construct validity and reliability of the SCIBI. A series of multilevel regression analyses showed higher age of the client to be negatively associated with assertive control. Job experience, level of education, type and sex of staff predicted interpersonal behaviour. Also, intrapersonal staff behaviour, including critical expressed emotion, proactive thinking and self-reflection, predicted interpersonal behaviour. Conclusions: The SCIBI can be used to identify staff intrapersonal and interpersonal behaviour towards clients with ID and challenging behaviour. Results obtained with the SCIBI can provide new directions for individual client treatment plans and staff training programmes.