This study examined the pedagogical content knowledge (PCK) of two groups of physical science teachers identified as experienced and novice chemical demonstrators. Science teachers, as do other teachers, use their pedagogical content knowledge to make decisions on how to teach very specific subject matter topics to students of various ages and abilities . Given the importance of this knowledge system for effective science teaching, this study also examined the influence of an intensive chemical demonstration workshop (a NSF summer institute program) on fostering pedagogical content knowledge growth among a group of eight novice chemical demonstrators. A clinical interview, consisting of a critical-stop task and a semi-structured interview, served to probe the experienced and novice chemical demonstrators' PCK. The critical-stop task required subjects to view videotaped chemical demonstrations and stop and discuss the tape at segments they perceived contributed to effective or ineffective chemical demonstrating . The clinical interview focused on teachers' pedagogical knowledge of demonstrating density and air pressure. Domain, taxonomic, componential, and theme analyses and several quantitative content analyses were conducted on the verbal data . The data indicated that experienced chemical demonstrators possess quantitatively greater and qualitatively richer pedagogical content knowledge for demonstrating basic chemical concept.s than novice chemical demonstrators. Experienced demonstrators, unlike novices, possess a large body of knowledge about chemical demonstrations, chemical demonstration variations, and accompanying inquiry strategies for presenting subject-matter topics to middle school students. The study further showed that the two-week chemical demonstration workshop produced an increase in the number of chemical demonstrations and demonstration variations novice chemical demonstrators could discuss on the targeted concepts, density and air pressure. The novice demonstrators also became more cognizant of the complexity of several chemical demonstrations, how these complexities could interfere with learning, and how simplified variations of these chemical demonstrations could promote science concept learning. After the workshop, novices' verbalizations also contained fewer references to pedagogically unsound chemical demonstrations on the targeted concepts. Many of these changes brought novices closer to the characteristics of experienced chemical demonstrators' PCK.