The purpose of this study was to investigate the development of preservice science teachers' knowledge structures in the domain of oxidation and reduction chemistry. Knowledge structures were elicited through video-recorded semi-structured interviews before and after the unit of instruction, and analyzed using a visual flow map representation. Paralleling these interviews, the preservice teachers were tasked with diagnosing middle school students' scientific understandings. Data analyzed quantitatively and qualitatively showed large variation in knowledge structure complexity across the preservice teachers, strong correlations between measures of knowledge structure and diversity (as defined by the Shannon Wiener diversity index), and the development of more balanced knowledge structure representations. For most preservice teachers, their diagnostic scores of the middle school students showed a small increase.