A series of rigid interpenetrating network foams (IPNFs) based on a rosin-based polyurethane (PU) and a crosslinked epoxide resin (ER) were prepared by a simultaneous polymerization technique. The morphology, mechanical properties, thermal stability, and changes in the chemical structure during the thermal degradation process of the rigid IPNFs were investigated by scanning electron microscopy (SEM), compressive testing, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR). The SEM micrographs showed that the cell structure of the rigid IPNFs became less homogeneous with increasing ER content. The brittleness of the cell walls increased as the ER content and the cure time of the rigid IPNFs increased. The compressive strength of the rigid PU/ER IPNFs increased to a maximum value and then decreased with further increase in the ER content. Similar behavior was observed for the elastic modulus. This behavior was related to the nonhomogeneous cells and more brittle cell walls for the rigid IPNFs with high ER content. The TGA data showed that the thermal stability of the rigid PU foam increased with the addition of increasing levels of ER, due to the better thermal stability of the ER compared to that of the PU. With the exception of the ER alone, a two-stage weight-loss process was observed for all these rigid IPNFs and for the PU foam alone. The FTIR analysis suggested that the first stage of weight loss was due to the degradation of the polyol-derived blocks of the PU, and the second weight loss stage was governed by both the degradation of the ER component and that of the isocyanate-derived blocks of the PU. (C) 2000 John Wiley & Sons, Inc.