Research on the effectiveness and mechanisms of mindfulness training applied in psychotherapy is still in its infancy (Erisman & Roemer, 2010). For instance, little is known about the extent and processes through which mindfulness practice improves emotion regulation. This experience sampling study assessed the relationship between mindfulness, emotion differentiation, emotion lability, and emotional difficulties. Young adult participants reported their current emotional experiences 6 times per day during 1 week on a PalmPilot device. Based on these reports of emotions, indices of emotional differentiation and emotion lability were composed for negative and positive emotions. Mindfulness was associated with greater emotion differentiation and less emotional difficulties (i.e., emotion lability and self-reported emotion dysregulation). Mediational models indicated that the relationship between mindfulness and emotion lability was mediated by emotion differentiation. Furthermore, emotion regulation mediated the relationship between mindfulness and both negative emotion lability and positive emotion differentiation. This experience sampling study indicates that self-reported levels of mindfulness are related to higher levels of differentiation of one's discrete emotional experiences in a manner reflective of effective emotion regulation. (PsycINFO Database Record (c) 2016 APA, all rights reserved)