Sixty British primary school children aged 9-10 and their teachers took part in an experimental teaching programme, designed to improve the quality of children's reasoning and collaborative activity by developing their awareness of language use and promoting certain ‘ground rules’ for talking together. Children's subsequent use of language when carrying out collaborative activities in the classroom was observed and analysed, and effects on their performance on Raven's Progressive Matrices test of non-verbal reasoning were also investigated. Comparative data were gathered from children in matched control classes. Qualitative and quantitative analyses of discourse showed a marked shift in target children's use of language in accord with the aims of the teaching programme, and demonstrated that adherence to the ground rules helped groups solve the reasoning test problems. Children's individual scores on the Raven's test also improved. These findings support a sociocultural view of intellectual development and confirm the value of explicitly teaching children how to use language to reason.