Diffusion of a weak acid (mandelic acid) was studied as a function of pH in highly concentrated W/O emulsions (gel-emulsions). The main characteristic of highly concentrated emulsions is their high internal phase volume. Gel-emulsions were prepared using aqueous solutions at different pH values. The surfactant was of the ethoxylated alcohol type and the oil, a hydrocarbon. Diffusion studies were performed by putting in contact two emulsions of the same composition except for the diffusion molecule and in some experiments the pH of the medium. The concentration of this molecule was determined as a function of the distance to the plane of contact. Diffusion coefficients were determined by means of numerical solutions of Fick＇s laws with appropriate boundary conditions. It has been found that the pH of the medium strongly influences diffusion. This has been attributed to the modification of the partition coefficient of mandelic acid between the continuous and dispersed phases caused by the different solubility of the neutral and ionized species. The results allow confirmation of the contribution of the partition coefficient to the diffusion in gel-emulsions and an explanation of the mechanism by which diffusion takes place in these systems.