A new free-volume model for tracer diffusion coefficients in supercritical CO2 and liquid water is proposed in this work. It embodies the concepts of free volume and activation energy. The free volume is calculated by the Carnahan-Starling expression, and the necessary effective hard sphere diameter was taken from a previous publication. The model is explicit, straightforward, contains one parameter per system (activation energy), and only requires temperature, solvent density, solute molecular weight, and the solvent LJ constants. The validation of the model was accomplished with a large database, comprehending 289 systems with 5485 data points, and achieved only 3.56% of error. The fine predictive capability of the new expression was also demonstrated. The correlations of Wilke-Chang, Tyn-Calus, He-Yu-Su, Zhu et al., and Dymond were adopted for comparison, but provided much poorer results and/or prediction values. A spreadsheet for the calculation of D-12 is given in Supplementary data. (C) 2012 Elsevier B.V. All rights reserved.