Constitutive equations are derived for the isothermal viscoelastic response of glassy polymers at small strains. The model employs the concept of temporary polymeric networks for shear relaxation and the diffusion mechanism for volume recovery of compressible materials. Adjustable parameters are found by fitting experimental data for poly(methyl methacrylate) and polycarbonate in uniaxial tensile tests. It is demonstrated that the stress-strain relations correctly predict nonmonotonic changes in the specific volume observed in creep tests.