Replicating a finding now common in the literature, the present study revealed a significant difference between the performance of men (M = 19.66; SD = 5.34; SK = 0.52) and the performance of women (M = 14.85; SD = 6.06; SK = -0.38, Cohen's d = 0.90) on the Mental Rotation Test (Vandenberg & Kuse, 1978). In an attempt to identify determinants of the observed sex differences, hypotheses related to MRT item types (structural, mirror, occluded, and non-occluded) first suggested by Voyer (Voyer & Hou, 2006) were tested. Although the results in part supported the notion that different types of items may contribute to the production of sex differences, the bulk of the sex differences remained unexplained. In terms of factor structure, it is argued that the MRT is best conceptualized as a consistent set of non-independent items all contributing to the sex differences. Furthermore, neither the MRT as a whole nor any of the proposed subsets of items were found to be sex biased. In fact, the performance of men was significantly superior to that of women on all of the test's items. Finally, the sex differences were not found to be related to the degree of angular disparity in correct response choices. This last finding suggests that the sex differences observed on the MRT may not be primarily related to the ability to rotate objects mentally. (Contains 4 tables.)