In-situ spectroscopic ellipsometry is used for the dynamic study of thermally perturbed thin polystyrene films, swollen with n-octane or n-decane. The thermal evolution of the swollen films reveals pronounced changes both in equilibrium and kinetic properties. Upon vitrification, a kinetically arrested swollen matrix isobserved. This phenomenon is related to the excess energy term in the chemical potential of the solvent, associated with the non-equilibrium status of the swollen polymer matrix. The swollen polystyrene – n-decane system shows no measurable compression when exposed to hydrostatic pressures up to 50 bar, neither above or below the glass transition temperature. A slight compression is observed for a glassy polystyrene film in pressurized water. This compression corresponds to partial relaxation of excess free volume. The results are of importance for membrane applications in which penetrant-swollen polymer membrane layers are operated under high pressures.