The yield stress of polyacetal (POM) decreases monotonically with the incorporation of thermoplastic polyurethane (TPU) elastomer in POM/TPU blends as would be anticipated. However, the impact strength of the resultant POM/TPU blends increases initially up to 30% TPU and thereafter decreases with the addition of TPU. Stress relaxation measurements in simple extension were carried out for POM and its blends with 10, 20, and 30% TPU at a constant temperature (30°C). Rate of loss of the relaxation modulus was found to be a nonlinear function of time. It has been demonstrated that the stress relaxation modulus values measured at different strains can be superimposed by a shift along the logarithmic time axis to yield master curves of modulus over an extended time period. It has also been found that while it is possible to determine, at any strain, relaxation curves covering an appreciable time range, the demarcation of linear and nonlinear behavior ranges of stress could not be done for these materials as all the strain values chosen in our experiments were in the region of linear behavior. © 1993 John Wiley & Sons, Inc. Copyright © 1993 John Wiley & Sons, Inc.