A current English education policy is to encourage all state primary schools to become academies: state-funded, non-selective, and highly autonomous establishments. Primary schools have been able to opt-in to academy status since 2010 and academies now account for twenty-one per-cent of the primary sector. This paper investigates the causal effect of voluntary academy conversion on primary school assessment outcomes, and on entry-year intake composition. Unlike existing evidence focused on earlier academies formed from failing secondary schools, no evidence is found of an academy conversion effect on attainment for the average pupil, although pupils with special educational needs do perform better in reading tests after academy conversion. There is no evidence that academy conversion affects the composition of the entry-year intake.