The present article derives from two isolation studies performed for the European Space Agency (ESA) as simulations of space travels, lasting for 4 and 9 weeks. The aim was to study how interpersonal relations were affected by time, and how individual characteristics related to conflicts and tension among crewmembers. A broad battery of methods was used, including video-recording, peer-ratings and self-reports. In both studies, group-functioning declined in the middle of the isolation and towards the end of the stay. Dominance and task motivation seemed to be important characteristics for compatibility between crewmembers, and negative relations established early in the isolation remained stable over time. These findings have implications for composition, training, and support of crews for extended spaceflights.