The present study investigates the empirical separability of mathematical (a) content domains, (b) cognitive domains, and (c) content-specific cognitive domains. There were 122 items representing two content domains (linear equations vs. theorem of Pythagoras) combined with two cognitive domains (modeling competence vs. technical competence) administered in a study with 1,570 German ninth graders. A unidimensional item response theory model, two two-dimensional multidimensional item response theory (MIRT) models (dimensions: content domains and cognitive domains, respectively), and a four-dimensional MIRT model (dimensions: content-specific cognitive domains) were compared with regard to model fit and latent correlations. Results indicate that the two content and the two cognitive domains can each be empirically separated. Content domains are better separable than cognitive domains. A differentiation of content-specific cognitive domains shows the best fit to the empirical data. Differential gender effects mostly confirm that the separated dimensions have different psychological meaning. Potential explanations, practical implications, and possible directions for future research are discussed.