The existence of a minimum in the surface tension (gamma)-concentration (C) curve of surfactant solutions is investigated. Equilibrium surface tension and solution conductivity (kappa) of anionic surfactant (SDS) solution and mixed anionic-nonionic surfactant (SDS-NP(EO)(40)) solutions are measured. Minima are observed in the gamma-log C curves. The critical micelle concentration (CMC) inferred from the K-C data does not match the bulk concentration at which the minimum in the gamma-log C curve occurs, but corresponds to the one at which surface tension reaches the constant equilibrium value. Polyoxyethylene nonionic surfactants CmEn are mixed and solutions show no minimum in the gamma-log C curves, whereas minima in gamma-log C curves for solutions of C10E8 and C12E8 mixing with a trace of n-dodecanol are observed. Therefore, the existence of a minimum or a negative peak in the gamma-log C profile may imply that the surfactant is a mixture, but no negative peak in the gamma-log C profile does not imply that there is no impurity in the surfactant or the surfactant consists of only one component. A clear break with no negative peak in the gamma-log C profile may be a better criteria for surfactant purity, but is still not a guarantee.