Designing high school chemistry assessments is a complex and difficult task. Although prior studies about assessment have offered teachers guidelines and standards as support to generate quality assessment items, little is known about how teachers engage these supports or enact their own beliefs into practice while developing assessments. Presented in this paper are the results from analyzing discourse among five high school chemistry teachers during an assessment item generation activity, including assessment items produced throughout the activity. Results include a detailed description of the role of knowledge bases embedded within high school chemistry teachers' pedagogical content knowledge and the processes used to enact these knowledge bases during planned formative assessment design. Implications for chemistry teacher professional development are posited in light of the findings as well as potential future investigations of high school chemistry teacher generation of assessment items.