The influence of values, problem awareness, and personal norm on willingness to cooperate were studied. The choice situation was seen as a social dilemma and willingness to reduce personal car use as an intention to cooperate since it requires sacrificing immediate personal gains in order to reduce long-term collective environmental costs. Data were collected through a survey to 2500 car owners in Sweden. The survey assessed, value orientation, problem awareness, personal norm, and willingness to reduce personal car use. A hierarchical model, of the effects of values, awareness, and norms on the willingness to cooperate, was tested with path analysis. Data supported the hypothesized model. Values and problem awareness influenced personal norm, which in turn influenced willingness to reduce personal car use. The results clearly showed the importance of personal norm for willingness to cooperate in a social dilemma.