An investigation has been carried out to determine the extensional mechanical behavior of a viscoelastic material when it is subject to an infinitesimal displacement field superposed on a finite steady deformation. Experiments were conducted on a thin strip of a polyester‐based polyurethane at a temperature of 77°F (±3°F). It is shown that the relaxation modulus of this material can be expressed as a product of two functions, one of which depends only upon the elongation ratio and the other only upon time. This form is applicable for basic‐state elongation ratios1⩽λ⩽1.2and for a period of 2 hr. It is noted that the separable form is highly suspect for longer time intervals.