This paper reports on a sustained interdisciplinary effort between applied linguists and chemistry faculty to create and validate writing assessment instruments for an advanced-level Write Like a Chemist course, one component of a much larger interdisciplinary project. The article describes a multiple-year effort to form valid analytic and holistic assessment instruments to be used by chemistry faculty to assess the writing performance of chemistry majors. Emphasis is placed on the joint contributions of applied linguists and chemists in (a) the identification of meaningful writing criteria, (b) the development of assessment scales with distinct score points and descriptors, (c) socialization sessions that prepared chemists to help build the assessment instruments, and (d) the validation of assessment instruments with other chemists. Outcomes exemplify the mediating role that applied linguistics can play in the design of a discipline-specific course, instructional materials, and assessment instruments that support the development of disciplinary expertise. The results also demonstrate the positive consequences of crossing disciplinary boundaries for both subject-area faculty and applied linguists