The demixing diagram of the 6ΦC12/NF system has been determinedby surface tensiometry. It ispossible to fit approximately both the first cmc from the regularsolution theory with a pair interactioncoefficient equal to 2.2 and the second cmc from a mass balance of thesurfactants between the solutionand the mixed micelles. But the regular solution theory accountsneither for the coordinates of the“eutectic”point on the first cmc curve nor for the compositions in the demixingzone.