The paper presents a study concerning primary teachers' ideas related to the particulate nature of substances and their explanations of changes of states. The study took place in the context of an in-service training course of primary teachers. Research findings suggest that teachers hold misconceptions or alternative ideas, a number of which are similar to those of students, while these particle ideas were found to affect their explanations of change of states. In addition, the effectiveness of a specially designed intervention was tested by implementing pre/post assessment tests. Post intervention, both teachers' ideas and explanations were found significantly improved. Implications for science education are discussed.