The formation of micelles of Tween-20 and Brij-35 as well as of SDS, Tween-20, and Brij-35 mixed in different proportions in aqueous medium has been physicochemically investigated. The critical micellar concentration (CMC), micellar aggregation number, counterion binding by micelles, micellar polarity, free energies of micellization and interfacial adsorption, and entropy of micellization have been evaluated by conductometric, tensiometric, and fluorimetric measurements. The solution composition has been found to have a complex say on the measured physicochemical parameters. The enthalpies of micellization of both Tween-20 + Brij-35 and SDS + Tween-20 + Brij-35 mixed surfactant systems have been found to be negligibly small. Attempts to understand the properties of mixed micelles (composition, mutual synergism, component activity coefficients, and CMC) have been made with the help of the propositions of Clint, Rubingh, and Rubingh and Holland. The mixed binary and ternary systems can be adequately described by these theories. Copyright 1998 Academic Press.