In response to extensive research exposing students' poor understanding of the particle theory of matter, this article argues that the conceptual framework within which the theory is introduced could be a limiting factor. The standard school particle model is characterized as operating within a "solids, liquids, and gases" framework. Drawing on an analysis of scientific ideas on matter and research into students' understanding, issues arising from the framework are identified which could contribute towards students' well known difficulties. The analysis leads to the proposal for a particle model based within the framework of the concept of a substance. Results from two exploratory studies using the substance-based particle model with children (ages 9-10) in two contrasting elementary schools in England are then reported. After a short teaching intervention with a class in each school, individual interviews were held with a sample of 12 students from each class. Data were collected on students' understanding of substances coexisting in different room temperature states and phenomena involving changes of state and mixing. The results gave useful feedback on the specification of the model and its teaching. Overall the students' engagement with the particle ideas was encouraging and suggests a larger scale testing of the substance-based model is merited. (Contains 3 figures and 4 tables.)