In this study, the effects of visualization tools on argumentation skills, knowledge acquisition, and motivation during learning with a hypermedia learning environment were examined. Participants in this experiment had to complete an argumentation task on environmental issues by using a hypermedia learning environment as resource. In one condition, participants were provided with a graphical mind mapping tool in order to complete an argumentation task. In a second condition, a two-columned text editor has been given. A control group received no argumentation task and no corresponding support device. Results suggest that a graphical argumentation support device can enhance learners’ motivation, but has no influence on knowledge acquisition or quality of arguments, whereas both argumentation support devices led to superior results compared to the control group. Overall, results reveal that the assignment of an argumentation task to hypermedia learning environments was an effective instructional strategy that led to enhanced knowledge acquisition compared to learning without an argumentation task.