Mutually penetrating polymer networks were prepared from polyurethane and polyurethane acrylate. Small angle X-ray scattering was used to examine microheterogeneities in mutually penetrating polymer networks and their variation with composition. From experimental results the average square fluctuation of electron density was calculated in these networks; scattering intensity, extrapolated to zero angle, thickness of the transition layer between two phases forming the networks, specific internal surface, the size of heterogeneity regions and average radii of maximum regions of heterogeneities in networks were determined. It was shown that mutually penetrating polymer networks are characterized by high intensity scattering, which is evidence of structural heterogeneity. Heterogeneity regions which are inclusions of a network into the matrix of the second, depend on network composition and vary most markedly in the region of average composition, which is due to phase inversion. The thickness of the transition layer is 20–40 Å.