Abstract The phase behavior of different fatty acids (FA) in pseudo‐ternary mixtures [(water)/(FA) + (alcohol)/(oil)] was studied. The Winsor I‐III‐II transitions of ionized compounds were attained by varying the FA composition through the formulation scan. The middle‐phase volume, the pH of the optimum system, and the partition coefficient increased with the chain length of FA. Conversely, the acid partition coefficient allows calculation of the Gibbs free energy of transfer of a CH 2 group from oil to water, which is found to be +0.87 J/mol.