Two studies (Study 1: n = 137; Study 2: n = 192) were conducted to investigate how upper-elementary students’ affect during small group instruction related to their social-behavioral engagement during group work. A circumplex model of affect consisting of valence (positive, negative) and activation (high, low) was used to examine the relation of affect to social loafing and quality of group interactions. Across both studies, negative affect (feeling tired or tense) was associated with higher rates of social loafing. Neutral to deactivated positive affect, such as feeling happy or calm, was positively related to positive group interactions, while deactivated negative affect (tired) was negatively associated with positive group interactions. Follow-up cross-lagged analyses to examine reciprocal relations suggested that positive group interactions altered affect on subsequent group tasks, but affect was not related to changes in positive group interactions. These quantitative findings were supplemented with a qualitative analysis of six small groups from Study 2. The qualitative analyses highlighted the reciprocal and cyclical relations between affect and social-behavioral engagement in small groups.