The effect of various compatibilizing compounds on the viscoelastic behaviour of semi-interpenetrating polymer networks (IPNs) based on cross-linked polyurethane and poly(n-butyl methacrylate) was investigated. It was established that in many cases, because of the specific chemical structure of compatibilizing compounds, the modified system reveals only one maximum of mechanical losses instead of two for initial semi-IPNs. This effect may be called compatibilization throughout the whole volume of the initially phase separated system. The disappearance of two maxima, revealing only one shows that the segregation degree in the IPN diminishes and in some cases miscible systems are formed.