The study investigates the reasoning paths followed by 74 prospective preschool teachers to support specific claims regarding three physical phenomena (boiling, evaporation and condensation) and one chemical phenomenon (combustion). The instrument was a paper-and-pencil test that included four tasks, one for each phenomenon, describing a situation within an argumentative framework. Results highlight significant difficulties of prospective preschool teachers in understanding physical and chemical phenomena, which emerge through their reasoning paths and their characteristics. Relevant implications for preschool science education are also discussed.