Achievement motivation is considered a prerequisite for success in academic as well as non-academic settings. We studied sex differences in academic and general achievement motivation in an adult sample of 338 men and 497 women (ages 18–70 years). Multi-group covariance and means structure analysis (MG-CMSA) for ordered categorical data was used to establish the location of possible sex differences, i.e., on the level of the latent factors or on the level of the observed items (i.e., sex-related item bias). Five of the 28 achievement motivation items showed severe bias with respect to sex, exemplifying the usefulness of MG-CMSA in locating the source of sex differences. The Academic Achievement Motivation scale consisted of two latent factors: Dedication and Persistence. Sex differences were observed for the factor Dedication only, with women showing more dedication towards their academic work than men. The General Achievement Motivation scale consisted of five latent factors: Pressure, Accomplishment, Work Approach, Future Orientation, and Competition. Sex differences were significant for the factor Future Orientation, with women contemplating less about the future than men, and a trend towards significance (p = .06) was observed for the factor Competition, with women being less actuated by competitive motives than men. These results suggest that sex-related item bias merits attention in achievement motivation research, but that men and women still differ in aspects of achievement motivation when biased items are eliminated from the analyses.