Purpose: There is speculation that high rates of mental illness among Reservists returning from deployment to Iraq and Afghanistan may be due to the challenge of reintegrating into civilian life. We aimed to examine the postdeployment social functioning of Reservists and to explore the relationship between adverse postdeployment experiences and subsequent mental ill health.Methods: A sample of 4,991 UK military personnel who had deployed to either Iraq or Afghanistan were asked about their postdeployment experiences with a particular focus on their levels of social integration, perceived support from the military, and civilian employment. All participants were asked to complete a series of validated measures of mental health.Results: Compared with Regular personnel, Reservists were more likely to feel unsupported by the military and to have difficulties with social functioning in the postdeployment period. Perceived lack of support from the military was associated with increased reporting of probable posttraumatic stress disorder (PTSD) and alcohol misuse. Low levels of non-military postdeployment social support and participation were associated with increased reporting of common mental disorder, probable PTSD, and alcohol misuse.Conclusions: Many Reservists find the transition from military deployment to civilian life difficult. Differences in postdeployment experiences may explain some of the increased rates of mental ill health among Reservists.