This paper presents a study concerning Greek primary school teachers' (n = 162) ideas about the particulate nature of matter and their explanations of physical phenomena. The study took place during an in-service training course where the effectiveness of a specially designed intervention was tested. A key feature was an approach based on the concept of a substance and its states rather than "solids, liquids, and gases". Pre-intervention, the teachers held misconceptions similar to those of pupils. Also, there seemed to be some relationship between the teachers' particle model ideas and their explanations of phenomena. Post-intervention, the teachers' descriptions and explanations were found to be significantly improved, with almost zero correlation between pre- and post-intervention scores. Implications for science education are discussed. (Contains 10 tables and 3 figures.)