Poly(cross-styrene)-inter-poly(cross-methyl methacrylate) interpenetrating polymer networks (IPNs) were prepared by simultaneously photo-cross-linking an anthracene-labeled polystyrene and photopolymerization of methyl methacrylate (MMA) monomer in the presence of ethylene glycol dimethacrylate (EGDMA) used as a cross-linker. The cross-linking reaction of these two polymer components is mutually independent and is drivable by irradiation with 365 nm UV light. The photopolymerization of MMA monomer exhibits an autocatalytic behavior due to the heat of polymerization and was analyzed by using a nonlinear kinetic model. As the reaction yield exceeds a certain threshold, the mixture undergoes phase separation. The time-evolution of the PMMA domains observed under a laser scanning confocal microscope follows a power law with an exponent approximately equal to unity. The domain growth was spontaneously frozen by the reactions and was terminated in a stationary bicontinuous structure whose characteristic length scales vary linearly with the kinetic parameters of the autocatalytic reaction. A general relationship for the kinetics of phase separation induced by this autocatalytic reaction was found over a large range of cross-linker concentration. Finally, the roles of the reaction autocatalysis in phase separation of polymer mixtures are discussed and compared to other cases of reaction-induced phase separation.