Type III microemulsions composed of anionic surfactant, non-ionic surfactant, water, oil, and sodium chloride were studied as a function of the anionic/total surfactant ratio. The activity coefficient of each of the five components in the microemulsion phase was determined. The activity coefficients of the surfactant components in the microemulsions were also computed on a surfactant-only basis. There is substantial negative deviation from ideality of mixing between the dissimilar surfactants. Regular solution theory provides a poor description of the surfactant interaction. In order to test the practical utility of using anionic/nonionic surfactant mixtures in enhanced oil recovery, systems containing alcohol were studied to cover the entire range of surfactant compositions. The optimal salinity and solubilization parameter for the mixtures were found to be similar to the values for systems containing the single surfactants of which the mixture is composed.