Polyurethanes (PU) based on poly(butylene adipate) [PU(PBA)] and poly(oxypropylene) [PU(PPG)] polyols are employed as a graft agent to prepare interpenetrating polymer networks of urethane-modified bismaleimide (UBMI) and the diglycidyl ether of bisphenol A (Ep) (UBMI/Ep graft-IPNs). The UBMI is introduced and partially grafted to the epoxy by PU graft agents, and then simultaneous bulk polymerization technique is used to prepare the graft-IPNs. All the PU graft agents were characterized by infrared (IR). The tensile strength of both the UBMI/Ep graft-IPNs with PU(PBA) and PU(PPG) graft agent systems increased to a maximum value with increasing UBMI content in the system and then decreased with further increasing the UBMI content. For both kinds of PU with various molecular weight in the UBMI/Ep graft-IPNs, the Izod impact strength increased with the UBMI contents increasing. The better compatibility of PU(PBA)-based UBMI/Ep, graft-IPNs led to higher impact strength. On the contrary, the fracture energy (G(IC) value) of the resultant UBMI/Ep graft-IPN showed that the UBMI/Ep graft-IPN with PU(PPG) graft agent had much higher de value than that with PU(PBA) graft agent. The morphology of this IPN with PU(PBA) graft agent exhibited a homogeneous one-phase system, while the UBMI/Ep graft-IPN with PU(PPG) graft agent showed a two-phase morphology with the UBMI particles dispersed in the epoxy matrix. (C) 2000 Elsevier Science Ltd. All rights reserved.