The aim of this research was to explore 11 to 12-year-old children's understanding of the mass of objects made from different materials, and the measurement of this attribute. In depth task-based interviews were conducted with six children attending six different schools; five were in Year 6 and one in Year 7. The children were found to hold alternative conceptions about mass, which they understood as being synonymous with weight. These conceptions appear to stem from confusion between the heaviness of an object and the heaviness of the material (its density) from which an object is made. It is recommended that measurement of mass in the Australian Curriculum: Mathematics be integrated with 'Chemical Sciences' in the Australian Curriculum: Science.