Sex differences on the Dutch WISC-R were examined in Dutch children (350 boys, 387 girls, age 11-13 years) and Belgian children (370 boys, 391 girls, age 9.5-13 years). Multi-group covariance and means structure analysis was used to establish whether the WISC-R was measurement invariant across sex, and whether sex differences on the level of the subtests were indicative of sex differences in general intelligence (g). In both samples, girls outperformed boys on the subtest Coding, while boys outperformed girls on the subtests Information and Arithmetic. The sex differences in the means of these three subtests could not be accounted for by the first-order factors Verbal, Performance, and Memory. Measurement invariance with respect to sex was however established for the remaining 9 subtest. Based on these subtests, no significant sex differences were observed in the means of the first-order factors, or the second-order g-factor. In conclusion, the cognitive differences between boys and girls concern subtest-specific abilities, and these sizeable differences are not attributable to differences in first-order factors, or the second-order factor g.