Vapor-phase clusters from ethanol-water liquid mixtures of varying compositions, generated by sampling the binary vapor in equilibrium with the liquid mixture, were examined with the aid of a time-of-flight mass spectrometer. The variation of the mole fraction of the alcohol in the cluster beam with the liquid mole fraction was found to be identical to that of the surface concentration obtained from surface tension measurements. The results also compare well with those obtained from neutron reflection. The mass spectrometric method of determining surface compositions of liquid mixtures is more direct and model independent and is applicable over the entire range of compositions. © 2001 Elsevier Science B.V.