On the basis of a generalized Frumkin-Damaskin isotherm, synergistic interactions between mixed surfactants adsorbed at fluid interfaces are discussed in terms of a mutual interaction parameter. By use of experimental sigma-log c curves of sodium dodecyl sulfate, oxethylated decanol (5EO), and two homologous (N-n-alkyl-N,N-dimethylammonio)acetic acid bromides (at pH = 7 predominantly betains), the adsorption parameters of these surfactants were determined, including their self-interaction parameters. By use of these adsorption parameters, an additional interaction parameter was determined, which characterizes the interaction between two different surfactants. This additional interaction parameter is small for mixtures of sodium dodecyl sulfate and the nonionic surfactant. It becomes considerable for mixtures of sodium dodecyl sulfate and (N-n-alkyl-N,N-dimethylammonio)acetic acid bromides, which indicates an additional synergistic attraction between the interacting molecules in the adsorption layer. The additional interaction parameters depend remarkably on the mixing ratio and correlate to the depression of the critical micelle concentration of these mixtures.