Dimensional comparisons (i.e., comparing own performances across domains) may drive an increasing differentiation in students' math and verbal self-concepts over time, but little longitudinal research has directly tested this assumption. Using cross-sequential data spanning Grades 1-12 (N = 1069, ages 6-18, 92% White, 2% Black, 51% female, collected 1987-1996), this study charted age-related changes in the role of dimensional comparisons in students' ability self-concept formation. It used three types of self-concept measures: peer comparisons, cross-domain comparisons, and no comparisons. Results indicated that the increase in students' use of dimensional comparisons in self-evaluations substantially contributed to the increasing differentiation in students' math and verbal self-concepts over time. Findings highlight the importance of dimensional comparisons in the development of students' ability self-concepts.