Background: Staff working within secure services for people with intellectual disabilities (ID) are likely to work with sexual offenders, but very little attention has been paid to how they think about this sexual offending behaviour. Method: Forty-eight staff working within secure services for people with ID were recruited and completed the Attribution Style Questionnaire in relation to the sexual offending behaviour and challenging behaviour of men with mild ID. Attributions towards challenging behaviour and sexual offending were compared and relationships between level of ID and seriousness of the sexual offence were explored. Results: The results indicated that staff attributed sexual offending as more external to the staff group than they did for challenging behaviour. Sexual offending behaviour was also seen as more stable, and less controllable by people with ID than was challenging behaviour. Sexual offending was also attributed as more uncontrollable by the staff group than challenging behaviour. There was a significant negative correlation between general intellectual functioning and several attributional dimensions regarding sexual offending, but not challenging behaviour. Sexual offending that was coded as more serious was attributed as universal and uncontrollable by the staff group. Conclusions: The differences between staff attributions regarding challenging behaviour and sexual offending potentially relate to the decision-making processes involved in deciding whether or not to involve criminal justice agencies when someone with ID commits a sexual offence. Further research within this area is warranted.