- Asphalt mixtures are composite materials that consist of asphalt binder, air voids and aggregate particles that vary by orders of magnitude in size. Most constitutive models of asphalt mixtures are formulated based on macroscopic measurements. However, little effort has been spent in the past in determining the relevance of these macroscopic measurements to actual material response at the microstructural level. This paper presents an overview of a viscoelastic-viscoplastic model for asphalt mixtures that was developed previously by the authors. The model's parameters were obtained in this paper by analysing triaxial repeated creep and recovery tests conducted at different confining and axial stresses. The microstructure characteristics of asphalt mixtures were determined by measuring aggregate physical characteristics, three-dimensional orientations of aggregates and air void distribution. The relationships of model's parameters with these microstructure characteristics are discussed in this paper. The results are used to draw conclusions in regard to the influence of microstructure characteristics on asphalt mixture response in terms of hardening, softening and dilation. These results have also provided insight in regard to the suitability of some of the macroscopic measurements in reflecting the actual changes in the material microstructure during deformation. Copyright 2010 Inderscience Enterprises Ltd.