We study adsorption at planar liquid-vapor interface of surface active binary mixtures and test three well-known models for the composition of surface phase. The models were originally presented by Guggenheim [Thermodynamics, 3rd ed.; North Holland: Amsterdam, 1967], Eberhart [J. Phys. Chem. 1966, 70, 1183], and Laaksonen and Kulmala [J. Chem. Phys. 1991, 95, 6745]. These are compared to results for model fluids from density functional theory (DFT). The model of Laaksonen and Kulmala is in best agreement with DFT calculations. Surface mole fraction of the solute component from the Guggenheim model exceeds one for a mixture with high surface activity. The failure of the Guggenheim model is also evident in our calculations for water-methanol, water-ethanol, and water-n-propanol mixtures.