We examine the effect of n-octanol (C(8)E(0)) on the phase behavior of the ternary microemulsion system;H2O/n-octane/pentaethylene glycol mono-rt-octyl ether (C(8)E(5)) at constant 1:1 water-to-oil volume ratio. The addition of C(8)E(0) shifts the three-phase regime to lower temperatures and distorts it. The shift of the three-phase regime can be explained by assuming that in the microemulsion a mixed amphiphilic film is formed, which becomes more hydrophobic as the alcohol concentration is increased. The distortion of the three-phase body is caused by a larger solubility of C(8)E(0) in n-octane than that of the C(8)E(5) monomers. From a quantitative analysis taking this effect into account, one obtains the relative proportions of C(8)E(5) and C(8)E(0) in the mixed film. Furthermore, the amphiphilic mixture (C(8)E(5) + C(8)E(0)) displays an increased efficiency compared to the pure components, which is attributed to synergism. Measurements of the interfacial tension between bulk water- and oil-rich phases of the quaternary microemulsion system support this interpretation. The interfacial tension passes through a minimum both on either changing the temperature or the C(8)E(0) content when the curvature of the film is tuned through zero. Interestingly, the synergism lowers the absolute value of the minimum interfacial tension of the mixed amphiphiles below that for an equivalent pure component.