There are a number of industrially important reactions in which a sparingly soluble solid reacts with a liquid-phase reactant with one of the products precipitating out as a solid. If the particles of the precipitate are in microphase dimensions, i.e. less than the diffusion length of the system, the solubility of the solute is increased by the microphase action of the product in the vicinity of the solid phase. The principal objective of the present work is to describe such 'autocatalytic' microphase action by developing a model which simultaneously considers reaction, crystallization, and microphase processes. The chief conclusion is that the growth kinetics and initial crystal size distribution of the precipitating solids have a profound influence on the reaction rate. Earlier models were deficient in that only microphase action was considered with limited effects of nucleation. The theoretical conclusions reported here are sought to be experimentally validated in Part II of this study.