Computer modeling and simulation reveals a grain size- and composition-dependent behavior of phase coexistence around the morphotropic phase boundaries in polycrystals of ferroelectric solid solutions. It shows that the width of phase coexistence composition range increases with decreasing grain sizes; phase-coexisting domain microstructures effectively reduce elastostatic, electrostatic, and domain wall energies; and grain boundaries impose internal mechanical and electric boundary conditions, which affect the phase-coexisting domain microstructures in the grains and give rise to the grain size effect of phase coexistence. (c) 2008 American Institute of Physics.