We have measured the equation of state surface for the mixtures poly(4-hydroxystyrene) + ethanol, and + tetrahydrofuran below 40 MPa. The results obey a superposition principle that makes it possible to build-a master curve once the p-V-T surface is known at a given pressure. The master curve depends on a single system-dependent parameter that, for the two systems studied, takes almost the same value. The results have been analyzed with a lattice fluid model that includes the existence of hydrogen bonds. Even though the model describes the composition dependence of the excess volume, it underestimates the pressure dependence of the density. While the inclusion of hydrogen bonds in the model has a negligible influence on the predictions for the system with tetrahydrofuran, it improves significantly the predictions for the system with ethanol.