The internal/external (I/E) frame of reference model (Marsh, "Am Educ Res J" 23:129-149, 1986) conceptualizes students' self-concepts as being formed by dimensional as well as social comparison processes. In the present study, the I/E model was tested and extended in a sample of elementary school children. Core academic skills of reading, writing, and math were related to corresponding and non-corresponding self-concept facets to determine the onset of dimensional contrast effects. School achievement and domain-specific academic self-concepts of 1,114 students from grades 1 to 3 were assessed. Negative paths were found for math achievement on reading self-concept and for reading achievement on math self-concept in the third grade. Math achievement was not associated negatively with writing self-concept. Positive influences were found within the verbal domain for writing achievement on reading self-concept from grade 1 onward. The results suggest a broad interpretation of the I/E model in which contrast as well as assimilation effects are possible. Factors influencing the dimensional comparison processes are discussed.