As part of a larger longitudinal project on the assessment of preschoolers' social-emotional development, children's social information processing (SIP) responses to unambiguous hypothetical situations of peer provocation were assessed for 298 four-year-olds from Head Start and private childcare settings. Measurement focused on emotions children would feel during these situations, and their behaviour response decisions. Participants most often chose sad and angry emotions, and socially competent and passive behaviours. Relations were found between sad emotion and socially competent behaviour choices, as well as between angry emotion and aggressive behaviour choices. Sad emotion and socially competent behaviour responses contributed to variance in contemporaneous and later school adjustment and kindergarten academic readiness. There was evidence that the contributions of sad emotion responses were mediated by those of socially competent behaviour choices. Results bolstered calls to include emotion in SIP measures, supported predictive validity for this SIP measure in a large representative sample of preschoolers, and pointed to fruitful pathways for future research.