In order to examine hypothesized underlying neurocognitive processes in repetitive behaviour, children and adolescents (7-16 years) with autism spectrum disorder (ASD) and obsessive compulsive disorder (OCD) were compared on a range of executive function (EF) measures. Performance on neuropsychological tests assessing executive functioning showed a trend for children with ASD to perform poorly on tasks requiring generation of multiple responses, while children with OCD tended to demonstrate impairments on a task requiring inhibition. Parental ratings on a questionnaire measure of EF indicated impairments in both groups relative to controls. Relationships between questionnaire and performance measures of EF were generally weak. There was some limited support for a relationship between EF and repetitive behaviour, but effects tended to be small and variable across groups and measures. (Contains 4 tables and 1 figure.)