Preschool children's emotion knowledge was examined as a possible mediator of the link between their mutual positive emotional expressiveness with peers and peer acceptance. Data were collected from 122 preschool children (57 boys, 65 girls; 86 European American, 9 African American, 17 Hispanic, and 10 other ethnicity; M age = 57.61 months) over a period of 2 years. In year 1 observations were made of children's emotional expressiveness with peers, and children completed sociometric interviews. In year 2, children completed emotion knowledge interviews and sociometric interviews. Analyses revealed that children who expressed more mutual positive emotion with peers in year 1 were better liked by peers in year 2, after controlling for year 1 peer acceptance. Mutual positive emotion in year 1 was associated with children's emotion knowledge in year 2. Both year 1 mutual positive emotion and year 2 emotion knowledge made independent contributions to peer acceptance in year 2.