Staff reports of the communication acts taking place with 22 adults with intellectual disability were compared with video observations of the communication acts used by staff with 12 of these service users. The interactions were coded in terms of the form of communication used, the function of the act and the level of complexity. The results show that staff tend to underestimate their own use of verbal communication and overestimate their use of non-verbal communication. The findings also indicate a mismatch between the reported level of understanding of the service user and the level of complexity of the language used. Staff appeared unable to adapt their communication to the skills of the service user and an average of 45% of communicative acts were outside the reported understanding skills of the individual. The implications of these findings are discussed and possible explanations for staff behaviour are suggested.