The phase behavior of Brij-56/1-butanol/n-heptane/water is investigated at 30 degrees C with a [weight fraction of oil in (oil + water)] = 0.5, wherein (2) under bar -> 3 -> (2) over bar phase transition occurs with increasing W, (weight fraction of 1-butanol in total amphiphile) at low X (weight fraction of both the amphiphiles in the mixture) and a (2) under bar -> 1 -> (2) over bar phase transition occurs at higher X. Addition of an ionic surfactant, sodium dodecylbenzene suffortate, destroys the three-phase body and decreases the solubilization capacity of the system at different 8 (weight fraction of ionic surfactant in total surfactant). A three-phase body appears at alpha = 0.25, but not at alpha = 0.75 for the single system. No three-phase body appears with the mixed system at either alpha value. Increased temperature increases the solubilization capacity of the Brij-56 system; on the other hand, a negligible effect of temperature on the Brij-56/SDBS mixed system has been observed. Addition of salt (NaCl) produces a three-phase body for both single and mixed systems and increases their solubilization capacities. The monomeric solubility of 1-butanol in oil (S-1) and at the interface (S-1(S)) has been calculated using the equation hydrophile-lipophile balance plane for both singles- and mixed-surfactant systems. These parameters have been utilized to explain the increase in solubilization capacity of these systems in the presence of NaCl. (c) 2005 Elsevier Inc. All rights reserved.