The maximum product concentration of a molecular compound at the feed point in reaction crystallization is analysed by modelling. A model describing unsteady mass transfer and chemical reaction in strained, semi-infinite, fluid films is combined with models describing productivity constraints and reactant concentration changes during the course of a process. The maximum product concentration is taken as a first estimate of the maximum supersaturation and is used as a basis to discuss the product crystal size that may be produced. The result suggests that at certain conditions we may find smaller product crystals at decreased reactant concentrations. The maximum concentration varies during the course of a single-feed semibatch process and the highest value is not necessarily found in the beginning. The variation with time, depends on the choice of the reactant to feed, and the results provide an explanation to why the product crystal mean size may depend on the choice of feed reactant. Guidelines are proposed for how to decide on optimum reactant solution concentrations and for how to select the reactant to feed in a semibatch process. In addition it is suggested that larger crystals may be produced if the feed reactant concentration is low early in the process and is allowed to gradually increase with time in a controlled way. The term "programmed feed concentration'' is introduced.