Performance of recycled asphalt pavements depends, among other things, on the degree of mixing of old and new binders. One of the factors contributing to the mixing is diffusion, which increases the homogeneity of recycled binders. In this study, FTIR-ATR (Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance) has been applied to investigate influence of temperature, viscosity of the diffusion medium, and diffusant size and polarity on diffusion characteristics. Several substances, referred to as markers, have been monitored when diffusing through different binders. By careful selection of markers, the importance of diffusant size and polarity on rate of diffusion has been demonstrated. Diffusion has been modelled using Stoke-Einstein's equation and compared with experimental data obtained by FTIR-ATR. The equation has shown useful in explaining the effects of temperature, viscosity and molecular size on diffusion. (C) 2003 Kluwer Academic Publishers.