Research on cognitive load theory (CLT) has not yet provided facet-specific measures of cognitive load. The lack of valid methods to measure intrinsic, extraneous and germane cognitive load makes it difficult to empirically test theoretical explanations of effects caused by manipulations of instructional designs. This situation also imposes challenges to testing CLT as a theory. This paper critically reflects the conceptualisation of CLT's core concept and the implications for its operationalisation. In order to address some of the challenges we propose a complexity framework that allows the derivation of "a priori" estimates of mental load that go beyond CLT's notion of element interactivity. In a study we test hypotheses with regard to effects of the variation of sources for intrinsic cognitive load (increase of complexity within tasks) and the variation of sources for extraneous cognitive load (reduction of extraneous cognitive load between tasks) in three ability groups. Complexity-based estimates prove superior to element interactivity-based estimates of mental load in the prediction of performance outcomes. Results also indicate that individual differences in information-processing capacity determine to what extent complexity is reflected as cognitive load. In this respect the proposed framework extends the focus of CLT beyond the discussion of the role of prior knowledge and acquired levels of expertise. (Contains 2 figures and 3 tables.)