Although a large quantity of diffusion coefficient data exist for sub-and supercritical (sc) fluids, these values are not applicable to the conditions associated with new applications such as material processing. The hydrodynamic equation D/T = alpha eta(beta), where D is the infinite dilution binary or self-diffusion coefficient, T is the temperature and I is the solvent viscosity, is useful for predicting diffusion coefficients in liquid and liquid-like sc states. This equation is also valid in dense solvent mixtures, but generates less accurate predictions in solvent mixtures. This deviation between experimental value for the mixture solvent and the background value based on a single component solvent was found to be correlated with the excess solvent viscosity value. New experimental diffusion coefficient data and a predictive correlation including the gas-like sc region (that is, at high temperatures and/or low densities) will be required to make accurate predictions when applying sc technology to material processing.