Sibling relationships appear important in fostering young children's growing theory-of-mind skills, but the quality of sibling interactions has rarely been investigated directly in relation to children's mental-state awareness (e.g. as indexed by talk about perceptions, desires, feelings, cognitions, i.e. inner state talk; IST). This study explored individual differences in young siblings' IST in relation to (i) child characteristics (verbal ability and theory-of-mind task performance); (ii) family characteristics (sib-ship size; social disadvantage, maternal well-being); and (iii) the quality of the children's play with their siblings (frequencies of pretence and reciprocal play). A socially diverse sample of 111 2-year-olds was filmed at home playing with a sib (mean age =4.92 years; SD=1.67 years); these sessions were coded from video for reciprocal play and then transcribed and coded for IST and pretence. Verbal ability was assessed for both sibs; in addition, 2-year-olds completed theory-of-mind tasks. Age and verbal ability were correlated with frequencies of overall talk, IST, pretence and reciprocal play. (Theory-of-mind scores were also correlated with 2-year-olds' IST, but this relation fell below significance once verbal ability was controlled). Significant independent relations were found between IST and children's quality of play, even when age, verbal ability and overall rates of talk were controlled. Although there were no gender contrasts in mean rates or in the variety of IST, gender differences were found in their correlates: in particular, context effects were significantly more important for girls than for boys. Individual differences in children's IST reflect multiple factors, including the dynamic features of children's reciprocal play.