Vrentas and Vrentas (Eur Polym J, 34 (1998) 797) have provided a method of evaluating diffusion coefficients using free volume theory, wherein all parameters can be estimated from the available experimental physical data or best estimates from approximate correlations (e.g. from a knowledge of the density, isothermal expansion coefficient, etc.). This method is tested using data for diffusion of methyl methacrylate and butyl methacrylate monomers in mixtures of the monomer and both polymers above the glass transition, over a range of polymer weight fraction and temperature (above T-g) (Macromolecules, 31 (1998) 7835). It was found that the predictions are reasonable for the well-studied MMA/PMMA system (although the predicted effective activation energy was poor), but the a priori predictions were in significant error for the other systems. The Vrentas method can be used reliably for interpolation and extrapolation of limited data, although because the predicted activation energies are too low, care should be taken with extrapolation over a significant temperature range.