Formation of interpenetrating networks (IPN) is accompanied by microphase separation due to the appearance of thermodynamic incompatibility of growing polymer chains of different nature. The phase separation remains incomplete as a result of high viscosity of the reaction medium and diffusion limitations. As a result the phase regions in phase-separated networks are separated by an interfacial region. The chemical and physical factors are discussed that determine the conditions of incomplete phase separation, degree of component separation and fraction of an intermediate region. This region preserves the structure of the reaction system before the onset of phase separation and remains in the state of forced compatibility. The final structure of phase-separated IPNs is characterized by the presence of three conventional phase regions which differ in composition, each region being an IPN with molecular level of mixing. All three regions are thermodynamically nonequilibrium, but the degree of deviation from equilibrium is different for each part of the system.