This quasi-experimental study examined 42 high school introductory chemistry students' conceptual understandings of the particulate nature of matter (PNM) before and immediately after instruction. Two groups of students, who were taught by the same teacher, received one of two possible instructional interventions: Reform-Based Teaching (RBT) or Reform-Based Teaching with Multiple Representations (RBTw/MR). The RBTw/MR instruction differed from the RBT instruction in terms of the "frequency" of using multiple representations (visual, textual, oral) in relationship to the macroscopic phenomenon and the likely actions occurring at the submicroscopic level. Qualitative research methods, including open-ended questionnaires and interviews, were used to investigate and describe participants' conceptual understandings of the PNM over time. The findings indicated that before instruction all participants held a range of alternative conceptions about the aspects of the PNM. Post-instruction findings indicate that the RBTw/MR instruction was more efficacious in promoting a scientific understanding of the PNM than was the instruction without multiple representations. (Contains 4 tables and 5 figures.)