Background: Students' self-concepts play an important role in explaining achievement-related behaviour. According to the internal/external frame of reference model (I/E Model), students use two frames of reference to form their mathematical and verbal self-concepts. The model predicts that external comparisons will lead to a positive association, and internal comparisons to a negative association, between mathematics and verbal self-concepts. A test of these predictions is possible by asking students to rate their mathematical/verbal ability in relation to that of other students (external self-concept) or to their own ability in other subjects (internal self-concept). However, recent studies using this approach were unsuccessful in assessing self-concepts based on the internal comparison process. Purpose: The present study is a new attempt to test the central assumptions of the I/E Model with frame-specific self-concepts in order to explain the process of self-concept formation. Sample: A total of 674 students from 25 German secondary school classes (grades 7-10) participated in the study. 373 of the participants were female. Design and methods: The students reported their most recent grades in mathematics and German. Frame-specific self-concepts were assessed via questionnaires. The data were analysed using structural equation modelling. Results: The analyses of the data support the assumptions of the I/E Model. Positive effects of achievement on the corresponding external self-concepts and negative effects of achievement on the non-corresponding internal self-concepts were found. A strong negative relationship between the internal self-concept factors shows that the operationalization of the internal comparison process was successful. Conclusions: These results contribute to a better understanding of how students form self-concepts.