The purpose of this study was to investigate pre-service science teachers' understanding of surface tension, cohesion and adhesion forces by using computer-mediated predict-observe-explain tasks. 22 third-year pre-service science teachers participated in this study. Three computer-mediated predict-observe-explain tasks were developed and applied to collect data. In the first stage of each task, pre-service science teachers were required to watch video related to the aforementioned concepts. They were asked to write their predictions individually, and to justify or give a reason for their predictions. In the second stage, pre-service science teachers were required to watch the continued video carefully. They recorded their observations individually. In the last stage, the pre-service science teachers were required to reconcile any conflict between their own predictions and observations. Their written responses were analysed to investigate their understanding. Results indicated that pre-service science teachers had difficulties related to surface tension, cohesion and adhesion forces. The obtained results underlined that the computer-mediated predict-observe-explain task was an effective method to diagnose students' understanding.