Epistemological beliefs are considered to play an important role in processes of learning and teaching. However, research on epistemological beliefs is confronted with methodological issues as for traditionally used self-report instruments with closed items, problems with social desirability, validity, and capturing domain-specific aspects of beliefs have been reported. Therefore, a new instrument with open items has been developed to capture mathematics-related epistemological beliefs, focusing on the justification of knowledge. Extending a previous study, the study at hand uses a larger and more diverse sample as well as a refined methodology. In total, 581 mathematics students (Bachelor of Science as well as pre-service teachers) completed the belief questionnaire and a test for mathematical critical thinking. The results confirm that beliefs can empirically be distinguished into belief position and belief argumentation, with only the latter being correlated to critical thinking.