The frequency, intensity and variability of emotional experiences increase in early adolescence, which may be partly due to adolescents' heightened affective sensitivity to social stimuli. While this increased variability is likely intrinsic to adolescent development, greater mood variability is nevertheless associated with the risk of internalising psychopathology. Early adolescents (N = 58, ages 13-14) reported their social context and mood when prompted by a smartphone application. Valence, arousal, and their variability were compared across social contexts using multilevel regression models. Social contexts were defined by the presence of close others, peripheral others, both, or neither. Arousal was lower when alone. Valence was lower and more variable, and arousal was more variable when alone than in either close or peripheral company. This is the first time that level and variability of valence and arousal in adolescent affect have been shown systematically to differ for the same individual in different daily-life social contexts.