Mindfulness is a state of awareness comprising an attentional focus on the present moment and a nonjudgmental stance. It is associated with affective well-being and assumed to facilitate adaptive emotion regulation. To support this claim at the within-person level, we investigated associations between 2 mindfulness facets (present-moment attention and nonjudgmental acceptance), 2 emotion-regulation strategies varying in adaptiveness (rumination and reflection), and positive and negative affect in everyday life using data from 2 experience-sampling (ESM) studies. Study 1 consisted of N = 70 students who completed 54 prompts on average. Study 2 consisted of N = 179 middle-aged adults who completed 69 prompts on average. Results from both studies were highly consistent: The mindfulness facet nonjudgmental acceptance was more strongly related to less concurrent rumination, whereas the mindfulness facet present-moment attention was related to more concurrent reflection. As predicted, both mindfulness facets interacted with rumination in the prediction of changes in affect. When individuals were in a more mindful state, rumination was less strongly associated with increases in negative affect, and was less strongly associated with decreases in positive affect. However, mindfulness interacted with reflection in the prediction of changes in affect in an unexpected way: At higher levels of nonjudgmental acceptance, reflection was no longer associated with changes in affect. Together, these results suggest that emotion regulation strategies can be more or less adaptive depending on the level of mindfulness. They also accord with the proposal that mindfulness inhibits maladaptive emotion regulation and its impact on affective well-being. (PsycInfo Database Record (c) 2020 APA, all rights reserved).