The traditional technique for the experimental characterization of the shear stress relaxation modulus by applying a step shear strain was investigated using flow visualization. A high-speed camera was employed in conjunction with cone-and-plate and parallel-disk fixtures of a Rheometric Scientific Advanced Rheometric Expansion System rheometer. In the nonlinear region the true shear strain imposed on a polyethylene melt deviates considerably from the targeted strain. The main source of the deviation is the wall slip of the polymer melt. The presence of wall slip reduces significantly the range of strains for which the strain-dependent relaxation modulus can be determined for the linear polyethylene melt. Errors associated with the control of the motion of the tool which introduces the shear strain are also documented. (C) 2001 The Society of Rheology.