The authors present a model for program-wide assessment of the effectiveness of writing instruction in a chemistry laboratory course. This model, which involves collaboration between faculty from chemistry, the Writing Across the Curriculum (WAC) program, and the Faculty Center for Teaching and Learning, is based on several theories and established practices from science education and WAC. Multidisciplinary experts scored writing samples and debriefed their findings. The quantitative and qualitative feedback prompted the modification of writing objectives and the realignment of course assignments. The interdisciplinary and collaborative process described could be implemented in a wide range of courses to provide insights on curricular effectiveness and tailor writing assessments to course writing objectives.