The ideas used by some 13 year old pupils to explain the properties of substances and their changes are analysed and discussed in this paper. A significant number of pupils use explanations which have been categorised as 'substantialisation of properties'. This categorisation has been justified in this paper on the basis of a study of the history of science. An analysis of the consistency with which students use this type of explanation, and factors favouring its use, has been carried out. © 1995 Kluwer Academic Publishers.