Reducing food waste is a major challenge in achieving a more sustainable food system. This research analyzes the psychosocial factors and cognitions that determine actual food waste behaviors in mass catering services. 216 customers of a French worksite cafeteria completed an online questionnaire based on the theory of planned behavior extended to moral norms. Over a period of four days, the quantity of food left by each respondent was weighed and linked to the answers. Findings indicate that food waste behaviors in mass catering setting are mainly drive by perceived behavioral control. Analysis of the underlying control beliefs suggests that interventions should focus on two specific aspects: improving food quality and making portion sizes more flexible.