This study measured high-school learners' cognitive load as they interacted with different web-based curriculum components, and examined the interactions between cognitive load and web-based concept learning. Participants in this study were 105 11th graders from an academic senior high school in Taiwan. An online, multimedia curriculum on the topic of global warming, which lasted for four weeks, provided the learning context. After students worked through the curriculum, their feelings about the degree of mental effort that it took to complete the learning tasks were measured by self-report on a 9-point Likert scale. An online test and the flow-map method were applied to assess participants' concept achievements. The results showed that curriculum components such as scientific articles, online notebooks, flash animations and the online test induced a relatively high cognitive load, and that a lower cognitive load resulted in better concept achievement. Also, students appeared to adopt different learning approaches that were corresponding to different levels of cognitive load. (Contains 9 tables and 1 figure.)