This study contributes to research on assessment centers' (AC) external construct-related validity by investigating a potential moderator of the relationship between personality and AC dimension ratings. On the basis of recent insights in person-situation contingencies we hypothesized that individual differences in people's perception of situational demands moderate the relationship between personality and conceptually related AC dimension ratings. The hypotheses were tested with 108 individuals in two leaderless group discussion exercises. Results confirmed the hypotheses for two of the three traits (i.e., Agreeableness and Conscientiousness). In particular, people high on these traits who identified the situational demands received higher AC dimension ratings. People high on these traits who did not identify the situational demands received lower AC dimension ratings.