Experimental liquid-liquid phase diagrams are presented for the multicomponent systems isooctane-benzene-(80 mass% methanol + 20 mass% water)-5 mass% isobutyl alcohol (2-methyl-l-propanol) and isooctane-benzene-(80 mass% methanol+20 mass% water)-15mass% isobutyl alcohol, at 298.15 K. The density and interfacial tension of conjugate phases of concentration located in the isothermal binodal have been determined at 298.15 K for the partially miscible systems: isooctane-benzene-methanol, isooctane-benzene-(80 mass% methanol + 20 mass% water), isooctane-benzene-(80 mass% methanol + 20 mass% water)-5 mass% isobutyl alcohol, and isooctane-benzene-(80 mass% methanol + 20 mass% water)-15 mass% isobutyl alcohol. The experimental tie-line data define the binodal or coexistence curve of the two studied multicomponent systems and depending on the initial isobutyl alcohol concentration the liquid-liquid phase diagram is either of type II, with low alcohol concentration, or type 1, with high concentration of alcohol, which is a clear indication that the solubility of the partially miscible systems is greatly enhanced via the co-solvency phenomenon. It is observed that both the density of each conjugate phase and the interfacial tension of each tie-line are valuable indicators of the degree of solubility of the multicomponent systems. Furthermore the experimental tie-lines data were correlated with the NRTL and UNIQUAC solution models with satisfactory quantitative results. (c) 2007 Elsevier B.V. All rights reserved.