In this work very accurate expressions for the tracer diffusion coefficient of hard sphere (HS) and real systems are proposed. The new HS model depends explicitly on the reduced density of solvent, and on the ratios of the diameters and masses of solute and solvent. It provides a very good representation of molecular dynamics data taken from literature: average absolute relative deviation, AARD = 4.44%. With respect to real fluids, the proposed model was developed according to Rice and Gray approach, and is based on the previous HS equation. The model involves only one parameter and requires temperature, solvent density, and solute and solvent molecular weight and LJ force constants (these are estimated as function of the critical temperature and molar volume). Results calculated for 309 systems and 5341 data points gave rise to AARD = 4.26%, and shows the model interprets equally well the diffusive phenomena of gases, liquids and SCFs. (C) 2010 Elsevier B.V. All rights reserved.