The transition from high school to college is associated with considerable life strain for adolescents, including higher reported levels of daily stress and negative affect (NA), and alterations in stress physiology have been linked to poor health. The purpose of this thesis was to use an ecological momentary assessment design to study associations between momentary experiences of negative affect and cortisol levels in a sample of adolescents transitioning to college. I also examined the potential moderating effects of two potential vulnerability or protective factors, alone status and perceived social support from friends. Adolescents provided salivary samples and completed paper-and-pencil diary reports of socioemotional experiences and alone status five times per day for three consecutive weekdays, as well as completed self-report questionnaires on perceived social support from friends. Within-person increases in momentary negative affect were associated with momentary cortisol reactivity. Alone status significantly moderated this association such that the association between momentary negative affect and momentary cortisol levels was only significant when individuals were with others and not when they were alone. Perceived social support from friends did not significantly moderate the within-person associations between negative affect and momentary cortisol levels. The findings add to our understanding of physiological correlates of socioemotional experiences, as well as contexts in which these associations may be exaggerated or attenuated. The findings inform our understanding of potential pathways by which physiological reactivity to socioemotional experiences may affect the health of adolescents as well as how prevention efforts could reduce potential poor health outcomes associated with heightened stress reactivity.