Pulsed NMR and small-angle X-ray scattering studies were performed on simple mixtures, semi-interpenetrating polymer networks (semi-IPN), and interpenetrating polymer networks (IPN) of polystyrene and polyurethane which were synthesized at different high pressures. It was found that, with increasing pressure, mixing of the components was enhanced. This was evidenced by the reduction in the size scale of the phases and/or the extent of mixing within the phases. In general, it was found that increasing the amount of cross-linking enhanced the mixing. Temperature-dependent small-angle X-ray scattering studies showed that the enhanced mixing of the linear chains could be reversed at elevated temperatures. For the semi-IPN's and IPN's, however, the cross-linking of the components either retarded the amount of coarsening that could occur or fully arrested the coarsening. However, for both the semi-IPN's and IPN's local coarsening processes were evident.