Research has shown that the average values for academic interest decrease during adolescence. Looking beyond such quantitative decline, we explored qualitative change of interest in the domain of mathematics across adolescence. Study 1 was based on a longitudinal data set (annual assessments from Grade 5 to Grade 9; N = 3,193). Latent variable modeling showed that the measurement coefficients of the latent variable of interest (intercepts, structural weights, and error variances) significantly differed across time points, indicating structural changes of the construct. Study 2 was based on interviews with adolescents (Grades 5 and 9, N = 70). Cognitive validation was used to explore differences in subjective concepts of interest across age groups. As expected, there were significant age-related differences, indicating a shift from an affect-laden concept in 5th grade to a more cognitively oriented concept in 9th grade. The findings suggest that developmental research should integrate quantitative and qualitative perspectives of construct change over time and pay more attention to issues of measurement invariance and qualitative changes of constructs over time. (Contains 4 tables and 1 footnote.)