The phenomenon of synergism in tension or adsorption for binary surfactants in water, for concentrations above and below the cmc, is studied using the nonideal adsorbed solution (NAS) model with one-parameter (beta(sigma)) regular solution model for interactions at the interface. Another parameter (beta( )(sigma)) is used for intramicellar interactions in the pseudophase separation model. Ranges of concentrations, compositions, and surface tensions are determined for which a mixture is synergistic below or above the cmc. Equilibrium tension data for aqueous C(12)E(8)/SDS (in 0.5 M NaCl) and our own data for C(12)E(5)/Triton X-100 were analyzed for tension synergism. Adsorbed surface densities were calculated from tension data and "maps" showing areas of tension or adsorption synergism are calculated for the first time. Although tension synergism is predicted when beta(sigma) < 0, adsorption synergism is possible even when beta(sigma) = 0. These results can be useful in designing surfactant formulations of controlled tension or surface composition with minimal amounts.