Background <p>Rates of stress and anxiety are alarmingly high in university communities, but most people do not receive treatment. Mobile health (mHealth) interventions show promise to improve psychological symptoms and increase access to interventions, but little is known about their effects in the moment. The present study evaluated the short-term impact of brief mHealth sessions to determine which intervention features are associated with the greatest momentary self-reported improvements.</p> Methods <p>Participants (<i>N</i> = 100 undergraduate students, graduate students, and university staff members) completed brief training sessions 1–2 times daily of MASK, a new mobile application for the university community that uses Cognitive Bias Modification for Interpretations (CBM-I) to shift anxious thinking patterns. Training sessions varied based on stressor domain/topic selected and writing requirements, among other features. Linear mixed effects models were used to test whether stressor domain or writing requirements predict post-training: (1) momentary affect, (2) reappraisal self-efficacy, and (3) emotion regulation self-efficacy.</p> Results <p>Self-reported improvement in state affect, reappraisal self-efficacy, and emotion regulation self-efficacy occurred for six out of eight stressor domains. Additionally, training sessions requiring less (vs. more) writing were associated with greater positive changes in affect, but not reappraisal or emotion regulation self-efficacy.</p> Conclusion <p>Stressor domain and writing requirements are associated with different in-the-moment cognitive and affective outcomes, pointing to the need to tailor mHealth programs to users’ specific needs and current stressors.</p>