Particulate polymer-in-polymer mezodispersions show a pronounced increase in the size of the dispersed particles during melt-phase annealing. Three ripening mechanisms have been proposed: Brownian coalescence, Ostwald ripening, and hydrodynamic coarsening. The modified Cahn-Hilliard equation predicts growth by Ostwald ripening and diffusion-induced coalescence. Simulations of this mechanism show a self-similar particle size distribution, but the distribution broadens with the increasing volume fraction of the minor phase. Hydrodynamic coarsening caused by concentration gradients and random Brownian forces has been simulated according to the hydrodynamic model. The simulations show that concentration-driven hydrodynamics have little effect on the particle size distribution. Experiments have been performed to investigate the relative importance of these ripening mechanisms for polybutadiene in a polystyrene system. (C) 2004 Wiley Periodicals, Inc.