The density profiles of liquid-gas interfaces of binary mixtures are determined by X-ray reflectivity measurements. The surface tension gamma is calculated from the measured data in the framework of the capillary wave theory. It is demonstrated that gradient theory can be used to investigate the van der Waals interaction between the different constituents of the liquid. The determination of Lermard-Jones constants and mixing rules for molecule-molecule interactions leads to a microscopic understanding of the liquid-gas interface. The refinement of the gradient theory to the data shows very good agreement when a liquid-gas interface according to a capillary wave roughness is used. (C) 2008 Elsevier B.V. All rights reserved.