Here the dilatational response of polymeric solids is considered. Such response is an important factor in the description of many dynamic situations including shock and acceleration wave propagation. Since such materials exhibit both instantaneous and equilibrium elasticity, as well as rate‐effects, it is assumed that polymers can be characterized as finite linear viscoelastic solids and their dilatational behavior is described in terms of an instantaneous and equilibrium hydrostat. These functions represent the bulk response to hydrostatic loads applied over very short and very long time periods; and here a technique for determining these functions from quasi‐static and ultrasonic wave propagation experiments is presented. To illustrate the results, the instantaneous and equilibrium hydrostats are given for the solid polymer, polymethyl methacrylate.