Recently, some interpenetrating polymer networks with good mechanical damping properties have been synthesized. However, the effect of morphology on this property has not yet been clearly elucidated. Herein, two polystyrene-polyurethane interpenetrating polymer networks, which were grafted using TMI [benzene-1-(1-isocyanato-1-methyl ethyl)-3-(1-methylenyl)] and HEMA (2-hydroxyethyl methacrylate), respectively, have been investigated, as model samples, by modulated-temperature differential scanning calorimetry and! by dynamical mechanical thermal analysis. The results indicate that there is a correlation between mechanical damping and both interphase content and the distribution of composition in the interphase region. The findings should provide valuable information for the design of future damping materials.