The nonideal adsorbed solution model (NAS) is extended to the air/water interface for determining the surface tension of aqueous binary surfactant mixtures with molecules of different size, surface activity, and nonideal interactions. The regular solution theory is used for modeling the nonideal interactions between the adsorbed molecules and in the micelles. Two mixing parameters beta(o) and beta(m) are used for fitting data for premicellar and micellar concentrations. This model can also be extended to account for molar area changes upon mixing in the monolayer or more complex nonideal interactions. Synergism in surface tension reduction efficiency can be predicted with this model. The ranges of mole fractions and concentrations for the existence of synergism can be determined. Tension data of two nonionic binary mixtures, C(12)E(8)/SDS (from the literature) and C(12)E(5)/Triton X-100, fit this model well. New tension data for C(12)E(5)/Triton X-100 are reported, and they are synergistic for a specific range of concentrations. Moreover, the surface coverages and compositions for the two nonionic mixtures are calculated with the NAS model. These calculations show that the larger molecules adsorb at the interface more favorably at low concentrations than at high concentrations.