The concentration dependence of the diffusion coefficients and the equilibrium isotherms of benzene, o-xylene, ethylbenzene, and chloroform in natural rubber membranes at 303 K were experimentally determined. The data were used to critically test the predictive capability of the Vrentas-Duda free volume model. It was found that although the model works well for some polymer-solvent systems such as toluene-polystyrene, the use of some of the parameters from pure component properties yields unacceptably low diffusion rates for the rubber-solvent systems studied. The parameters D-ol,1 and xi obtained from experimental zero-concentration diffusivity data, and parameter (V) over cap(1)* calculated from the solvent molecular geometries are needed to achieve good predictions. The diffusion coefficients described by the revised model can be used to predict quite well the breakthrough times of the rubber-solvent systems that were also experimentally measured by a permeation method.