The microphase structure of single polyurethane (PU) and acrylate networks as well as sequential interpenetrating polymer networks (IPNs), produced by the forming of a PU network in the presence of monomers of a penetrating network, was studied by small- and wide-angle x-ray analysis. It was established that each network component was of a two-phase structure consisting of disordered phase-separated microregions. the higher crosslink density of the acrylate network results in its higher heterogeneity. in IPNs, phase separation of a complex nature is realized: the PU matrix preserves some features of a single network structure, and the second component forms microregions 5-10 nm in size while retaining a certain level of interpenetra-tion of both network components. the microphase structure parameters of such systems are greatly dependent on the crosslink density of the penetrating network. This suggests the influence of a three-dimensional network of chemical bonds on the interdiffusion of branched fragments of the penetrating network and molecular chains of the matrix, one leading to the retardation of phase separation.