Given the importance of diagnostic assessment as a well-substantiated pedagogical strategy for use at various educational levels from kindergarten to the undergraduate level, we must consider its lack of implementation in the classroom. The implementation gap is especially severe at the tertiary level, with chemistry and other STEM (science, technology, engineering, and math) instruction being no exception. Yet, some tertiary instructors perform diagnostic assessment. How does this happen? What motivates, enables, and sustains these instructors in their implementation of diagnostic assessment? In this study we collect data on the practices of two tertiary chemistry instructors, including classroom observation, artifact collection, and interview. Analysis shows that these instructors employ several techniques that are consistent with the paradigm of research-based diagnostic assessment. Perceived motives behind the use of these techniques, as well as perceived barriers to further implementation of diagnostic techniques are reported and discussed. (Contains 3 tables and 6 figures.)