The influence of the co-surfactant on physicochemical properties of w/o soybean oil microemulsions (MEs) has been studied. In spite of the similarity in phase diagrams, the MEs display remarkable differences when examined by electrical conductivity, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and linear voltammetry. When different short-chain alcohols were employed as co-surfactants, together with sodium dodecyl sulfate (SDS) as surfactant, the DLS results indicated the systems to be monodisperse. Both the electrical conductivity of the MEs and the hydrodynamic radii of the droplets (R(H)) increased with water content while R(H) diminished as temperature increased, no aggregation or percolation of the droplets being observed. In comparison to w/o MEs prepared with 3-methyl-1-butanol, those prepared with 1-pentanol presented higher electrical conductivity and larger limiting currents at a Pt ultramicroelectrode for oxidation of the water occluded into the particles. Finally, from the electrochemical viewpoint the use of 1-pentanol is recommended, no advantage being gained by using any of the other tested alcohols.