This paper examines our understanding of private speech - speech that is not addressed or adapted to the interlocutor - from a Vygotskian perspective. The progressive acquisition of the planning function made possible by the interiorisation of language deserves special attention. Social and private speech is analysed longitudinally in ten pairs of children ut the ages of 4;6 and 6;6, in a play context. The results are relevant in two aspects. First, only ludic-emotional private speech (speech related to fantasy, play, or expressions of affect) decreases significantly with age, whereas planning private speech (describing actions directed to a goal)remains constant ut both ages. Second, a sequential analysis reveals that social speech categories, which are related to the expression of divergent perspectives, increase the probability that the partner will answer in a private manner. Therefore, the emergence of private speech seems to be related not only to the difficulty of the task but also to communicative conflict.