Thousands of studies show that mindfulness interventions improve cognitive, affective, stress, and health outcomes, but very little experimental work attempts to explain the mechanisms underlying this broad range of effects. This study uses a novel smartphone training paradigm to experimentally dismantle the components of mindfulness training and test their effects on affective and stress outcomes. Specifically, 153 participants completed one of three structurally equivalent 14-lesson interventions: (1) training in both monitoring and acceptance (Monitor+Accept), (2) training in monitoring only (Monitor Only), or (3) active control training. Before and after the intervention, positive affect was assessed in daily life. At post-intervention, cortisol and blood pressure stress reactivity and positive affective experiences were assessed in response to the Trier Social Stress Test. Early analyses show that, compared to Monitor Only and control training, Monitor+Accept training increased positive affect in daily life and during acute stress and reduced cortisol and blood pressure reactivity. This study provides the first evidence that acceptance training is a critical mechanism of mindfulness interventions for increasing positive affect and reducing biological stress reactivity. More broadly, this study demonstrates the value of brief smartphone-based mindfulness interventions for reducing stress and increasing well-being.