Knowledge about students' conceptions is one of the requisite components of pedagogical content knowledge. A keen awareness of students' alternative conceptions provides teachers with information about prospective difficulties students may incur as they make attempts to learn more accurate scientific representations of critical concepts. In this study, we investigated elementary school teachers' understanding of their students' alternative conceptions about change of states and dissolution. The subjects were 152 elementary school teachers and 529 sixth graders in Korea. A conceptions test and the test of the understanding about students' conceptions were administered in order to examine students' alternative conceptions and the teachers' awareness of their students' alternative conceptions, respectively. The effects of teachers' characteristics such as teaching experience, highest academic degree, science teaching efficacy, and views about teaching and learning (i.e., constructivist and traditional) in relation to their awareness of students' alternative conceptions were also investigated. The results indicated that the teachers tended to overestimate the number of students with scientifically accepted conceptions. The teachers also did not possess adequate knowledge about the existence and the distribution of their students' alternative conceptions. It was found that teaching experience, highest academic degree, science teaching efficacy, and the level of teachers' adoption of a constructivist view about teaching and learning were not significantly related to the their awareness of students' alternative conceptions. It was found, however, that there is a significant relationship between the level of teachers' traditional view about teaching and learning and their awareness of students' alternative conceptions. Educational implications are discussed.