This study explored the controversy surrounding working memory: whether it is a unitary system providing general purpose resources or a more differentiated system with domain-specific sub-components. A total of 348 participants completed a set of 6 working memory tasks that systematically varied in storage target contents and type of information processing, as well as a set of 6 tests measuring general fluid intelligence and general crystallized intelligence. A structural equation modeling approach formalized and tested the theoretical expectations of the differentiation perspective on working memory. Visuo-spatial working memory was found to be more strongly correlated with general fluid intelligence than with general crystallized intelligence, and vice versa for verbal-numerical working memory. Additionally, general fluid intelligence was more strongly correlated with visuo-spatial working memory than with verbal-numerical working memory, and vice versa for general crystallized intelligence. These patterns of relationships supported the argument that working memory is not a simple unitary system, but can be differentiated in domain-specific components which are visuo-spatial working memory and verbal-numerical working memory. (Contains 3 tables and 3 figures.)