The viscoelastic nonlinear behavior of several base and polymer modified asphalts (PMA) has been studied in step-strain experiments. The polymers were poly(styrene-b-butadiene-b-styrene), poly(styrene-b-ethylene-co-butylene-b-styrene), poly(ethylene-r-vinylacetate) copolymers, and a linear low density poly(ethylene), which were chosen as representatives of the main categories of asphalt modifiers. Because of the complexity of the morphological structure of these materials, the relaxation modulus has only partial and qualitative similarities with that of melt or high concentrated solutions of entangled polymeric liquids. No time strain separability can be applied, and the relaxation experiments are conveniently described by means of the memory functions. These have been calculated both via a parametric fitting procedure and by interpolation algorithms. Results are presented, and a correlation between the PMA structure and the corresponding memory function is proposed for the investigated materials. (c) 2007 Wiley Periodicals, Inc.