A series of in situ sequential interpenetrating polymer networks (IPNs) of polyurethane (PU) and polystyrene (PS) were prepared at room temperature. The PU network was made from oligomeric polypropylene oxide, end-linked with an aliphatic triisocyanate. The PS network results from free radical photocopolymerization of styrene with a small amount of divinylbenzene. During synthesis, the homogeneous initial mixture segregates into co-continuous phases with no chemical bonds between them. However, the samples exhibit high optical transparency. The measurements of refractive index and equilibrium swelling in ethyl acetate gave the evidence of interpenetration, i.e. additional entanglements between unlike network chains. (C) 2004 Elsevier Ltd. All rights reserved.