A proposed method of determining the composition of mixed micelles in equilibrium with monomer of known composition is described. The systems were sodium α-sulfonated dodecanoyl ethyl esther (α-SR12Et) or sodium dodecyl sulfate(SDS)-polyoxyethylene 23 lauryl ether (Brij 35) un water and in 0.1M sodium chloride solution at 25℃. This technique applies the Gibbs-Duhem equation to the mixed micelles, which is treated as a pseudophase. This proposed methodology, which needs only critical micelle concentration data as a function of monomer composition, is applied to an anionic/nonionic surfactant pair. The calculated monomer-micelle equilibrium is found to be very similar to the much-used regular solution for nonideal systems.