The present study investigates the development of children's ability to make inferences about a peer's mental state. In this study 48 eight-year-old children, 49 ten-year-old children and 44 adults observed and analyzed short video sequences, extracts from a socio-cognitive interaction between two children working on a mathematical task. The participants inferred mental states of one of the videotaped children while answering questions. Half of the participants watched the different sequences with sound and the other half watched them without sound. The results show that ten-year-old children are nearly as skilled as adults in making inferences but that eight-years-old children make less relevant inferences. The access to verbal information influenced the inferences only for the eight-year-old group. (Contains 3 figures, 1 note and 7 tables.)