Interpenetrating polymer networks (IPNs) based on blocked polyurethane (BPU)/phenolic (PF) were prepared using simultaneous polymerization method. The IPN was prepared from BPU prepolymer with m-xylylenediamine as the chain extender and PF prepolymer using p-toluene sulfonic acid as a catalyst. From Fourier transform infrared spectra analysis, it was found that the major reactions in the BPU/PF IPN system are the polymerization of BPU/MXDA and the self-polymerization of PF. It was confirmed from scanning electron micrography that the BPU/PF IPN compatibility of the networks was improved, and the system was heterogeneous and more than one phase existed in the IPN. The tensile properties, flexural properties, impact, and hardness of the prepared IPN were studied. To obtain the best mechanical properties of IPN materials, the filler added in IPN materials has to be investigated. POLYM. ENG. SCI., 51:285-293, 2011. (C) 2010 Society of Plastics Engineers