Research has shown that how well people can differentiate between different emotional states is an essential requirement for adaptive emotion regulation. People with low levels of emotion differentiation tend to be more vulnerable to develop emotional disorders. Although we know quite a lot about the correlates of emotion differentiation, research on factors or interventions which could improve emotion differentiation skills is scarce. Here, we hypothesize, and study empirically, whether a mindfulness-based intervention (MBI) may impact the differentiation of negative and positive emotions. A within-subjects pre-, post-, and follow-up design involving experience sampling was used. At each phase participants reported their current emotions and mindfulness skills up to 40 times across 4 consecutive days using smartphones. Multilevel modeling showed a significant improvement in negative emotion differentiation postintervention and at 4 months of follow-up, and a significant improvement in positive emotion differentiation at 4 months follow-up. The improvement in negative emotion differentiation, however, was no longer significant when controlling for levels of negative affect. A time-lagged mediation model showed that posttreatment changes in mindfulness skills mediated subsequent changes in negative emotion differentiation, also when controlling for levels of negative affect. These results suggest that MBI is a promising approach to improve people's emotion differentiation skills. (PsycINFO Database Record (c) 2019 APA, all rights reserved).