College science education needs to foster students' habit of mind beyond disciplinary constraints. However, little research has been devoted to assessing students' interdisciplinary understanding. To address this problem, we formed a team of experts from different disciplines to develop interdisciplinary assessments that target introductory college-level science. We started our project by focusing on osmosis, a topic that involves knowledge from multiple science disciplines. We developed an instrument focusing on this topic and administered it to 3 classes of college students. A Rasch partial credit analysis showed that the items demonstrated satisfactory psychometric properties. The findings also revealed the differences between student's disciplinary and interdisciplinary understanding. The educational implications of the study were discussed.