The pseudo pair potential theory of mixtures developed by Salsburg1 has been tested by carrying out Monte Carlo calculations for hard sphere systems of radius ratios 5 : 3 and 3 : 1 at low and moderate densities. Comparison between the results obtained by use of the pseudo-potential model with those obtained earlier by direct Monte Carlo simulation studies2, 3 for such mixtures shows that the model is not a good approximation. An explanation of the observed discrepancies is proposed.