We report a study of the surface tension of three binary liquid mixtures of molecular fluids, A microscopic mean field theory (MFT) has been used to calculate the theoretical results enabling the comparison with the experimental data, The mean field theory has been successfully used in the prediction of the surface properties of simple systems composed by quasi-spherical molecules, In the present study the MFT was able to reproduce the essential features of the interfacial properties of the systems CH4 + Kr, Kr + NO and CH4 + NO, The pure components were modelled by Lennard-Jones potentials with a set of intermolecular parameters taken from the literature for Kr and calculated from the fitting of the energy parameters (epsilon) to the surface tension, for CH4 and NO. In the case of the mixtures, it was found that reasonable agreement with experiment can only be obtained by allowing deviations from the Lorentz-Berthelot combining rules. For the CH4 + Kr system we used the binary energy parameter xi obtained through a fitting to the bulk properties; for the Kr + NO and CH4 + NO systems the binary parameter was adjusted to the interfacial properties.