This study investigated the interaction of disciplinary and interdisciplinary learning in a team-taught, first-year, interdisciplinary sustainability course. We surveyed (pre/post) both STEM (science, technology, engineering, and mathematics) and non-STEM majors (N = 241), assessing attitudes and content knowledge. Responses were analyzed using factor analysis, classical test theory, and Rasch analysis. Multivariate analyses of variance were performed to look for pre/post differences and differences between groups. Confirmatory factor analysis verified that the content knowledge items adequately assessed disciplinary and interdisciplinary understanding separately. Tests indicated an adequate item difficulty range, but also a need for more items at the higher end. All groups improved significantly over the semester. Slightly higher gains for STEM students and a correlation between disciplinary, and interdisciplinary learning might indicate a possible benefit of "disciplinary grounding." The instrument vetted through this study expands the number of validated sustainability content knowledge items. Further, it allows users to probe content knowledge along both interdisciplinary and disciplinary dimensions.