The psychometric properties of the systemizing quotient (SQ) developed by Baron-Cohen (2003) are investigated in three studies. Furthermore, we examine the notion that the ability to systemize should be independent of intelligence. In Studies 1 and 2, confirmatory factor analyses are used to examine the factor structure of the SQ. Study 3 examines the relationship between systemizing, mental rotation and intelligence. Studies 1 and 2 indicate that the SQ does not possess a unifactorial structure but is best considered as four related factors; Study 3 found that SQ was not related to intelligence, although mental rotation was. A four factor structure using fewer items was a better fit for the data than either the original version of the SQ or Wakabayashi et al.'s (2006) revised version. Overall these results support Baron-Cohen's view that SQ is not related to intelligence. Although mental rotation is correlated to SQ, it is not the main determinant of SQ. The problems of self-report measures are discussed along with the difficulties related to measuring systemizing.