This study explored preservice teachers' (n = 25) knowledge structures and their mastery of content knowledge in relation to their ability to diagnose the strengths and weaknesses of a fourth grader's videotaped explanations of a scientific phenomenon, i.e., molecular kinetic properties of air. Participants' knowledge structures were analyzed using flow maps before and following a unit of instruction on molecular kinetic properties of air. Participants' diagnostic scores were obtained before and after instruction. An additional diagnostic score used to explore relationships between knowledge structures and participants' abilities to apply understandings in a novel task was obtained following instruction. Findings include a significant increase in the mean number of diagnostic scores following instruction (t = 5.6; p less than or equal to 0.00) and a positive correlation (r = 0.58; p less than or equal to 0.02) between knowledge structure complexity and participants' ability to diagnose the child's thinking on a novel molecular kinetics task. Implications for teacher education programs are discussed.