Isothermal pressure relaxation as a function of temperature in two pressure ranges has been measured for polystyrene using a self-built pressurizable dilatometer. A master curve for pressure relaxation in each pressure regime is obtained based on the time-temperature superposition principle, and time-pressure superposition of the two master curves is found to be applicable when the master curves are referenced to their pressure-dependent T-g. The pressure relaxation master curves, the shift factors, and retardation spectra obtained from these curves are compared with those obtained from shear creep compliance measurements for the same material. The shift factors for the bulk and shear responses have the same temperature dependence, and the retardation spectra overlap at short times. Our results suggest that the bulk and shear response have similar molecular origin, but that longtime chain mechanisms available to shear are lost in the bulk response. (c) 2007 Wiley Periodicals, Inc.