The kinetics of solid state reactions has been reexamined to include solutions for mixed control by an interfacial process and diffusion. The Valensi-Carter, Ginstling-Brounshtein, and other quasi steady state models have been extended to include mixed control. Solutions were obtained for planar, cylindrical and spherical particles. Mixed control may occur at either the inner or outer interface for cylindrical or spherical particles, for which significantly different solutio ns are valid. Mixed control at the outer interface usually gives faster reaction than mixed control at the inner interface. The kinetics of these solid state reactions with mixed control is usually nearly independent of the product to reagent volume ratio.