The morphology of polyurethane-polystyrene (PU-PS) (60:40 by weight) interpenetrating polymer networks (IPNs), in which internetwork grafting via 2-hydroxyethyl methacrylate resides (HEMA) (1, 2.5, and 10 wt %, respectively) in the polystyrene networks has been studied by means of small angle X-ray scattering (SAXS), modulated-temperature scanning calorimetry (M-TDSC), and dynamical mechanical thermal analysis (DMTA) techniques. With increasing internetwork grafting, the average size of domains became smaller (SAXS data) and the degree of component mixing increased (M-TDSC and DMTA results). For the PU-PS (60 : 40 by weight) IPN with 10% HEMA, the DMTA tan delta -temperature plot showed a single peak. This DMTA result implied that the morphology of this PU-PS IPN is homogeneous. However, the M-TDSC data showed that three PU-PS (60 : 40) IPNs samples (with 1, 2.5, and 10 wt % HEMA, respectively) were phase separated. For the three IPN samples, the correlation length of the segregated phases, obtained from SAXS data based on the Deby-Bueche method, did not show distinct differences. With increasing internetwork grafting, the scattered intensity decreased. This study concluded that for these IPNs, SAXS is sensitive to the size of domains and component mixing, but no quantitative analysis was given for the component mixing. M-TDSC is suitable to be used to quantify the degree of component mixing or the weight fraction of interphases, and DMTA is sensitive to damping behavior and to phase continuity. However, DMTA cannot provide quantitative information about the degree of component mixing or the weight (or volume) fraction of the interphases.