Liquid-vapour coexistence and surface tension of binary mixtures is studied by Monte Carlo simulation. Both mixtures with a difference in the size parameters and mixtures with a difference in the energy parameters of the Lennard-Jones potential are investigated. The results for the surface tension and the coexisting liquid density are compared with the predictions of one-fluid models. The van der Waals model is found to be quite accurate, except in cases of strong interfacial adsorption. An interpretation of this accuracy is given, by developing a one-fluid model based on random packing of hard spheres. It is found that the predictions of this random-packing model are close to predictions of the van der Waals one-fluid model. The authors also present a simulation of the argon-krypton liquid-vapour interface.