Many polymeric materials, including structural adhesives, exhibit a nonlinear viscoelastic response. The nonlinear theory of Knauss and Emri (Polym. Engrg. Sci. 27, 1987, 87-100) is based on the Doolittle concept that the free volume controls the mobility of polymer mols. and, thus, the inherent time scale of the material. It then follows that factors such as temperature and moisture, which change the free volume, will influence the time scale. Furthermore, stress-induced dilatation will also affect the free volume and, hence, the time scale. However, during this investigation, dilatational effects alone were found to be insufficient for describing the response of near pure shear tests of a bisphenol A epoxy with amido amine hardener. Thus, the free volume approach presented here has been modified to include distortional effects in the inherent time scale of the material. The same was found to be true for a urethane adhesive. The small strain viscoelastic responses of the two materials have been determined from master curves of uniaxial and bulk creep testing at various temperatures The nonlinear free volume model, modified to include distortional effects in the reduced time, was incorporated in the ABAQUS finite element code via a user-defined material subroutine. For the epoxy, validation of the modified theory (a strain-based formulation of free volume) has been achieved through good agreement between the computational and exptl. results of butterfly-shaped Arcan specimens subjected to loadings ranging from near pure shear to shear with various amounts of superposed tension and compression. In addition to predicting the response under a variety of multiaxial stress states, the modified free volume theory also accurately predicts the formation and growth of shear banding, or regions of highly localized deformation, which have been found to occur upon continued loading of the epoxy. The urethane did not appear to exhibit any localized deformation over the range of temperatures at which it was tested. As a result, a stress-based modified free volume approach was required to model its multiaxial and temperature-dependent behavior. Although free volume was the unifying parameter for the two materials, the need for a stress-based and strain-based formulation of the free volume for the urethane and epoxy, resp., could not be reconciled at this time.