The effects of compatibilizing additive (oligo-urethane dimethacrylate) on the kinetics of IPN formation based on cross-linked polyurethane and linear polystyrene and its influence on the microphase separation, viscoelastic and thermophysical properties have been investigated. It was established, that 10, 20 mass% of OUDM introduced into the initial reaction system, prevent microphase separation of the system and lead to the formation of compatible IPNs, as follows from the data on light scattering. The viscoelastic and thermophysical properties of modified IPN (20 mass% OUDM) are changed in such a way that instead of two relaxation transitions characteristic for phase-separated system, only one relaxation transition is present. It is results of change the morphology of system. The position of this relaxation transition depends on the system composition and on the reaction conditions. (C) 2004 Elsevier Ltd. All rights reserved.