Organised knowledge structures (schemas) in long-term memory (LTM) alter the characteristics of working memory (WM). Therefore, tracing the content of WM during problem solving may provide a measure of levels of acquisition of corresponding schematic knowledge structures. In this paper, a proposed rapid approach to diagnostic assessment of levels of acquisition of organised domain-specific knowledge structures has been used as a means of optimising cognitive load by tailoring instructions to levels of learner expertise in an adaptive computer-based tutor in kinematics. Two alternative adaptation procedures were compared to the control condition without an adaptation. The first procedure used only results of learners' rapid test performance, and the second one combined rapid measures of performance with measures of mental load. The study indicates that the developed assessment instruments could be embedded into adaptive e-learning packages as diagnostic tools for tailoring instructional materials to changing levels of student expertise in cognitively optimal ways. Copyright © 2006 John Wiley & Sons, Ltd.