The compositions of mixed micelles formed in aqueous solutions of sodium dodecanoate and sodium perfluorooctanoate at different total surfactant concentrations and at three (0.33, 0.53, and 0.73) sodium perfluorooctanoate mole fractions were determined by the small angle neutron scattering technique coupled with the external contrast method. At each concentration, measurements were performed as functions of the solvent H2O/D2O composition in order to determine the micellar neutron scattering densities. The method described has allowed, at least for the cases under study, a direct determination of the micellar compositions, which, in the past, had been particularly difficult and the object of considerable debate. At all concentrations considered and within the experimental error, one kind of mixed micelle was always observed; these micelles were always richer in the component present in solution in greater proportion. An overall qualitative agreement between the present results and literature predictions based on regular solution theory was found, although a significant difference was noticeable at large sodium perfluorooctanoate concentrations, suggesting that further tests of the theory are required in order to include subtle interaction effects due to differences in the chemical nature of the surfactants.