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Emotion differentiation, the ability to make fine-grained distinctions between emotional states, has mainly been studied as a trait. In this research, we examine within-person fluctuations in emotion differentiation and hypothesize that stress is a central factor in predicting these fluctuations. We predict that experiencing stress will result in lower levels of emotion differentiation. Using data from a 3-wave longitudinal experience sampling study, we examined the within-person fluctuations in the level of emotion differentiation across days and months and tested if these fluctuations related to changes in stress levels. On the day-level, we found that differentiation of negative emotions varied significantly within individuals, that high stress levels were associated with lower levels of emotion differentiation, and that stress on 1 day negatively predicted the level of differentiation of negative emotions on a next day (but not vice versa). On the wave-level, we found a concurrent, but not a prospective relationship between stress and emotion differentiation. These results are the first to directly demonstrate the role of stress in predicting fluctuations in emotion differentiation and have implications for our theoretical understanding of emotion differentiation, as well as for interventions.