Micelle formation of a combination of hydrocarbon/fluorocarbon anionic surfactants-sodium n-tetradecyl sulfate (STDS) and sodium perfluorononate (SPFN) in water-was studied, paying special attention to the interaction between the two surfactants and the aggregation number, N, as a function of composition in the mixture (X-STDS). The critical micellization concentration (cmc) at each composition was determined at discrete temperatures by plotting the derivative specific conductance (partial derivativekappa/partial derivativeC) against the root of molarity (rootC) at 40 degreesC. This plot enabled us to determine the cmc even in solutions with addd salt (NaCl) and thus estimate the degree of counterion binding onto micelles (beta) for both single systems from the Corrin-Harkins relation. The micellar composition of STDS (Y-STDS) and interaction parameter were estimated on the basis of Rubingh's and our own theoretical equations, and the results obtained from the respective equations were compared. Although Rubingh's and our own equations resulted in differing phase diagrams, both suggested that the interaction mode between two surfactants and the state of formed micelles are remarkably different depending on the net mole fraction in the surfactant mixture (X-STDS) or Y-STDS. According to our equations taking into account the counterion effect on mixed micelle formation, that is, the degree of counterion binding, we found the existence of an azeotropic point and a region of mole fraction showing that almost completely demixing micelles are formed (0 < X-STDS < 0.2). Further, the aggregation number was measured by static fluorimetry using two pairs of probe P and quencher Q, and in addition, the microscopic environment inside micelles was investigated by the use of a pyrene probe. These fluorescence studies also revealed that the miscibility of the two surfactants and the properties of micelles are distinctive at X-STDS congruent to 0.2; above this mole fraction the two surfactants can form well-mixed micelles.