A problem encountered in quadratic electrostriction research in polymers is that viscoelasticity is usually not taken into account in theoretical predictions of intrinsic electrostriction whereas the apparent value obtained by experiment is a superposition of "true" electrostriction and spurious contributions from other cross-effects and viscoelasticity. A new approach is proposed to study how the "true" electrostriction constant is obscured by viscoelastic material behavior and to treat the influence of other relaxational phenomena in a systematic way. The method is purely macroscopic to ensure general applicability to different experimental situations and to avoid the use of any model or of assumptions on the molecular structure. Irreversible thermodynamics is used to derive the time-dependent equation of state of the viscoelastic dielectric. Once the dynamic equation of state has been obtained, all the tools that have been developed in the literature to study the irreversible thermodynamics of other systems, like fluids, for example, become directly applicable to the given problem.