The roles of reaction inhomogeneity in phase separation of polymer mixtures were described and summarized via two examples: photocross-link of polymer mixtures in the bulk state and photopolymerization of monomer in the liquid state. The reaction kinetics, the reaction-induced elastic strain and the phase separation kinetics were monitored respectively by UV-Vis spectroscopy, Mach-Zehnder interferometry and laser-scanning confocal microscopy. It was found that phase separation in the bulk state was strongly influenced by the elastic strain associated with the intrinsic inhomogeneity of the reaction, whereas the autocatalytic behavior of the polymerization plays an important role in the resulting morphology in the liquid state. These experimental results are discussed in conjunction with the morphology control of polymer mixtures by using chemical reactions.