The aim of this study was to explore pre-service primary teachers' understandings of the effect of temperature and pressure on the solid-liquid phase transition of water. In the study a survey approach was used, and the sample consisted of one-hundred and three, third year pre-service primary science teachers. As a tool for data collection, a test composed of five open-ended questions was used. In addition, semi-structured interviews were conducted with four purposefully selected students. All interview records were transcribed and analyzed with respect to the students' misunderstandings. The results show that there were common misunderstandings in the students' written responses. The statistical analysis indicated that the chi square statistic calculated is significant at a level of 0.05, suggesting that the observed frequency differences among categories are not due to chance. It is believed that the results of the study carry valuable knowledge for researchers and teacher educators. (Contains 3 tables.)