One-to-one interviews were administered to a sample of twenty female primary student teachers (PST), who were studying at the University of Cyprus. They were asked to describe the changes in macroscopic (colour, taste, volume, density, flammability) and microscopic (kind and movement of molecules) properties of substances when dissolving salt or sugar in water, when mixing water and alcohol, or when filtering or heating the respective water solutions. Analysis of the transcribed interviews showed that the majority of the PST exhibited perceptual rather than conceptual understanding of the particulate nature of matter and had difficulties to relate the observable macroscopic changes to the invisible molecular events (arrangement and movement of molecules). They stated instead that molecules share in observable properties of matter and combine together to give new molecules, without realising the changes in the structure and the properties of matter or without being able to distinguish physical from chemical changes. The prevalence and the diversity of the observed conceptions among PST indicate that the molecular constitution of matter is not adequately understood and that teaching materials and instructional interventions based on conceptual change should be designed and implemented both for teachers’ pre-service and in-service training, to avoid ‘diffusion’ of misconceptions within the primary classrooms. [Chem. Educ. Res. Pract. Eur.: 2000, 1, 249-262]