Preschoolers' theory of mind (ToM) was examined in relation to emotional features of their conflicts with siblings, using mothers as privileged informants. Fifty-four children aged 3 to 5 years and their 54 mothers took part. Children were given 10 standard false belief tasks and a standardized language test. Mothers completed questionnaires, rated vignettes, and kept a conflict diary to provide detailed data on siblings' conflicts, mothers' conflict attitudes, and mothers' use of mental state language to talk about children's disputes. Results revealed that children's ToM scores were significantly correlated with affective dimensions of their sibling disputes, including more frequent expression of positive emotion while disagreeing and less post-conflict distress. Logistic regression showed these associations were independent of age and verbal ability. Mothers' conflict attitudes were significantly correlated with the affective outcome of their children's disputes but not with the preschooler's level of ToM understanding. Findings are discussed in relation to possible reciprocal influences between ToM development and the growth of affectively positive and constructive conflict resolution skills.