This article deals with the determination of mechanical response of a nonlinear viscoelastic solid based on a Frechet expansion. A modified superposition principle suggested earlier by various authors is used to simplify the constitutive equation. It is then shown that the resulting stress‐strain equations are consistent with mechanical behavior of asphalt concrete. The inadequacy of linear viscoelasticity theory for these materials is also demonstrated. Practical laboratory determination of the kernel functions of the material is described.