ABSTRACT The association between Extraversion and positive affect is one of the most robust findings in the study of personality and emotion. Temperament models posit that the association is direct; instrumental models posit that the association is mediated by additional processes. Two experience sampling studies were conducted to test instrumental mechanisms that might underlie the effect. According to a mediation model, extraverts' greater social activity can account for their increased positive affect when compared to introverts. According to a person-by-situation interaction model, extraverts react more positively to social situations than do introverts, and this interaction can account for the association. Only weak support for the instrumental models was found; consistent with temperament models, a moderate direct association remained even after controlling for these effects.