We have obtained the p-V-T surface of the mixture acetone + poly(4-hydroxystyrene) over the intervals 298 less-than-or-equal-to T/K less-than-or-equal-to 328, 0 less-than-or-equal-to p/MPa less-than-or-equal-to 40, and 0.45 less-than-or-equal-to w less-than-or-equal-to 0.97, w being the weight fraction of acetone. The results obey a p-T-w superposition principle, making it possible to build a master curve when the pressure and the density are reduced with the isothermal compressibility and the density is at a reference pressure, respectively. The master curve depends on a single system-dependent parameter, which takes similar values for quite different mixtures. The master curve allows us to describe the pressure dependence of the density within experimental uncertainty. The bulk modulus of the system can be described quite accurately using a correlation inspired by a perturbation theory which assumes a spherical reference system. The data have been analyzed with two versions of a lattice-fluid model. One of the versions includes, in an approximated way, the existence of hydrogen bonds, both in the polymer and between the polymer and the solvent. However, almost no difference is found between the predictive ability of the two versions, at least for the p-V-T-w surface.