In this study, we investigated young children's (ages 7-10) spontaneously constructed or naive understanding of the particulate nature of matter prior to any formal instruction in the domain. Fifteen students were interviewed concerning their understanding of the macroscopic and microscopic properties of the states of matter (solid, liquid, and gas), as well as their macro/microscopic understanding of phase changes and dissolving. Children expressed ideas about states of matter which were categorized as macrocontinuous, macroparticulate, or microparticulate. Nine children (60%) stated beliefs about matter which were macroparticulate in nature, and three (20%) expressed microparticulate beliefs about matter. The three remaining children (20%) held macrocontinuous beliefs about matter. Furthermore, a substantial number of the children provided explanations of properties and processes which were consistent with those beliefs. These children's beliefs about matter were not fully and consistently developed across the spectrum of substances from continuous solids to particulate solids to liquids to gases. We speculate that children first develop local frameworks particular to different classes of substances and then slowly expand these frameworks to include a wide range of substances and their properties, as well as such processes as melting and freezing.