The critical micelle concentration (cmc) and the degree of counterion binding (beta) to micelles were determined as a function of composition in the mixture of dodecylammonium perfluoroacetate (DAPA) with dodecylammonium perfluoropropionate (DAPP) in water at 40 degrees C, The curve of cmc vs X(DAPP) (the mole fraction of DAPP in the surfactant mixture) was finely simulated by Rubingh’s equations, which enabled us to estimate the interaction parameter (W-R) based on the approximation of the regular solution; the curve fitting exhibited a slightly negative value (W-R = -0.131), indicating that the mixing enhances micelle formation due to a greater interaction than those of the respective pure systems. On the other hand, to the same mixed system were simultaneously applied our theoretical equations which were derived by taking into account the counterion effect, In contrast to the result estimated from Rubingh’s equations, the latter analysis indicated that the mixing of DAPA with DAPP seemed to be ideal, i.e., the interaction parameter (W-0) was estimated to be nearly equal to zero, The difference between WR and W-0 is directly ascribable to the contribution of counterions to the free energy of mixed micelle formation, The measured beta values were in an exact agreement with those calculated from the micellar composition estimated on the basis of our theory.