Students evaluate their achievement in a specific domain in relation to their achievement in other domains and form their self-concepts accordingly. These comparison processes have been termed "dimensional comparisons" and shown to be an important source of academic self-concepts in addition to social and temporal comparisons. Research on the internal/external frame of reference model (I/E model) has frequently found negative effects of students' achievement on their academic self-concept between different scholastic domains (mathematics and the language of instruction) that are interpreted as contrast effects of dimensional comparisons. There is mixed evidence with regard to whether negative contrast effects or positive assimilation effects occur when students compare their achievement in domains that are more similar. In this study, we extended the original I/E model with 3 science domains (biology, chemistry, and physics). Using structural equation modeling, we analyzed the domain-specific self-concepts, grades, and test scores of a representative sample of 9th-grade students in Germany (N = 20,050) across 5 domains. Mathematics, physics, and chemistry showed contrast effects to German, whereas small assimilation effects were found between mathematics, physics, and chemistry. This effect pattern was present for both grades and test scores. Achievement in mathematics and the language of instruction affected self-concepts in the sciences, whereas achievement in the sciences had no effect on self-concepts in other subjects. The results support the hypotheses derived from dimensional comparison theory that both contrast and assimilation effects can result from dimensional comparisons and that the 3 science subjects are affected differentially by these comparisons.