One hundred and thirty Greek primary school teachers participated in a study, where the effectiveness of a specially designed intervention on chemical changes was tested. The study took place in the wider context of an in-service training course where the key feature was an innovative approach based on the concept of a substance and its transformations, physical and chemical. In the present paper the focus is on the chemical transformations of substances. Pre-intervention, teachers were found to have a relatively limited ability in explaining chemical changes, which depends on the characteristics of the particular change, and they held a number of misconceptions similar to those of pupils. Post-intervention, teachers' descriptions and explanations were found to be significantly improved. Also, a relationship between teachers' particle ideas and their explanations was found. Implications for science education are also discussed.