This study among 244 employees and their colleagues working in various sectors investigated the dimensionality of self-ratings and peer-ratings of task and contextual performance, using the scales of Goodman and Svyantek (1999). By applying the multitrait-multimethod approach, we examined the degree to which responses to performance items are influenced by the trait (task or contextual performance) and the method factors (self- or peer-ratings). Results of confirmatory factor analyses showed that while the two performance dimensions (i.e., traits) can be differentiated, responses to their items are influenced by the method factor. Specifically, peer-ratings explain more variance in task performance, while self-ratings explain more variance in contextual performance. Moreover, the measurement of task and contextual performance is invariant across self- and peer-ratings. Finally, the positive relationships between task and contextual performance on the one hand and work-related flow on the other hand are of equal strength. These findings support the validity of the performance measure but also highlight some impact of the method factors.