The concept of matter is considered to be a fundamental concept for achieving scientific literacy, while students developing an understanding of its particulate nature is one of the prominent targets of chemistry curricula. The present study sought to examine how young children understand liquids (water) and solids (sugar) and their composition, and how these understandings progress through primary school. A sample of 108 children of four different year-groups (n = 27 5-year-olds, n = 27 7-year-olds, n = 27 9-year-olds, n = 27 11-year-olds) in the United Kingdom were interviewed using a structured interview protocol adopted from Nakhleh and Samarapungavan (1999). The findings suggest that young children start understanding materials in terms of their perceptual properties and their functions and properties in daily life, and as they grow older they attend to their composition and chemical properties. In terms of composition, children showed different conceptual progression patterns for sugar and water. They developed a better understanding of water, progressing from a continuous view of matter to the view that it is composed of invisible pieces of different shapes and sizes, while most of the children in all year-groups understood sugar as composed of visible pieces. A molecular understanding of matter did not emerge during the primary school years, something that is not surprising as primary school children in the UK are not formally instructed about the particle model. Even within the same year-group, there was significant variability in students' understandings of the materials and their progress did not appear to be a linear process.