Stress relaxation was measured after application of a double-step shear strain on a concentrated solution of polystyrene in chlorinated biphenyl. The first shear strain s1 was applied to the sample in a cone-plate sample holder of a relaxometer at time −t1, the second strain s2 was added at t=0, and then the stress was measured as a function of time t. Ranges of s1 and t1 were 1.71—20.5 shear units and 10—5000 sec, respectively, while s2 was kept constant at 1.88 s.u. These results, together with published results for strain-dependent relaxation moduli, were employed to assess a group of constitutive equations which use single integrals with respect to the time t′ to describe the strain history. It was shown that such constitutive equation could not in general describe the observed results, when invariants at t′ of the strain rate or the strain were employed in the response functions to represent the nonlinear behavior. However, in the special cases of relatively small s1 or large t1, constitutive equations of the single integral type described quantitatively the observed nonlinear behavior, when invariants of strain were employed in the response functions.