The aims of this cross-sectional case-control study were to explore the social information processing (SIP) patterns of children with reactive attachment disorder (RAD) and their association with behavioural difficulties. The sample consisted of 23 children with RAD aged 4-12 matched by age and gender with a comparison sample of typically developing peers. Children completed an assessment package investigating the encoding, interpretation, response access and response evaluation components of SIP and their intellectual functioning and verbal comprehension, while caregivers completed two instruments measuring child behavioural difficulties. The children with RAD displayed significantly more errors in encoding, interpretation and response evaluation than the comparison group. There was also a tendency for children with RAD to access more aggressive responses than comparison children. In addition, there were strong associations between aspects of SIP and child behavioural difficulties, with the strongest association between encoding and emotional problems. However, both intellectual functioning and verbal comprehension were significantly associated with behavioural difficulties suggesting these variables may impact on the relationship between SIP and behavioural difficulties. The overall findings suggest that children with RAD process social information in a biased manner and this is partially associated with behavioural outcomes. The strengths and limitations of the study are discussed in addition to suggestions for future research.