The 2I/E model is an extension of Marsh's (1986) classical internal/external frame of reference (I/E) model. Whereas the classical I/E model describes the formation of academic self-concept by the joint operation of social (external) and dimensional (internal) comparisons, the 2I/E model integrates temporal comparisons as an additional internal comparison process. In the 2I/E model, paths from achievement levels to self-concepts in the same subject are assumed to represent social comparisons, paths from achievement levels to self-concepts in another subject are assumed to represent dimensional comparisons, and paths from achievement changes to self-concepts in the same subject are assumed to represent temporal comparisons. The present research validated the 2I/E model in six empirical studies with a total of more than 20,000 students. Generally, there were strong positive social comparison paths, moderate negative dimensional comparison paths, and small positive temporal comparison paths. Furthermore, the relations of the 2I/E model were replicated with both grades and standardized test scores as achievement indicators (Study 1 and Study 2). Dimensional comparison effects were stronger between dissimilar subjects, like math and German, than between similar subjects, like German and English (Study 2 and Study 3). Also, the relations of the 2I/E model were shown when prior self-concepts were controlled for (Study 4). Overall, the 2I/E model seems to have the potential to enrich future theory and research on self-concept formation and comparison processes.