The surface tension of aqueous solution of decyl methyl sulfoxide (DeMS) and decyldimethylphosphine oxide (DePO) mixture was measured as a function of the total concentration of the surfactants and the composition in the surfactants. The compositions of the adsobed film and micelle were evaluated by applying thermodynamic equations to the surface tension and the phase diagrams of adsorption and micelle formation were drawn. The DeMS and DePO mixture showed a negative deviation from the ideal mixing in adsorbed film and a positive deviation in micelles, The activity coefficients of surfactant in the adsorbed film and micelle were calculated from the phase diagrams and the nonideality of mixing in the adsorbed film and micelle was clarified in terms of the excess thermodynamic quantities of adsorption and micelle formation. Judging from the negative values of the excess Gibbs free energy and area of adsorption and a positive one of the excess Gibbs free energy of micelle formation, it was concluded that the difference in head group between DeMS and DePO molecules causes a favorable packing in the adsorbed film and an unfavorable one in the micelle,