Crystals that present different polymorphic forms during their preparation pose challenging questions for science and industry. Given their relevant physical and chemical properties, understanding how the size distributions of polymorphs evolve during crystallization is important fundamentally and practically. Here, we propose a distribution kinetics model for the essential phenomena associated with polymorph crystallization processes. Included in the model are temperature effects for growth and dissolution coefficients, Gibbs-Thomson effects of particle curvature on equilibrium solubility, phase-transition energies (heats of solidification or vaporization), critical nucleus sizes for denucleation during coarsening, and interfacial energies. Numerical solutions of the governing population dynamics equations show that interfacial and transition energies, but not activation energies, are significant in causing the less stable dimorph to vanish. Evaporative crystallization and cooling and heating programs are potential ways to control polymorph separation.