Learning and understanding of science instructional material was examined from a cognitive load perspective. It was suggested that instructions can be difficult to learn if multiple elements of information need to be simultaneously processed through limited working memory. Diagrams were expected to reduce cognitive load by allowing students to process information using fewer elements in working memory than an equivalent text-based format. However, if instructional information could be processed serially, then working-memory load should be light, and both diagrammatic and text-based instructions were hypothesized to be equally effective. Two experiments using different chemistry instructions confirmed these hypotheses and so highlight the role of cognitive load factors in instructional design.