Differential capacitance of the electrical double layer at the water/nitrobenzene and water/1,2-dichloroethane interfaces was measured by an ac impedance technique. Potential difference across the diffuse layer in water, nitrobenzene and 1,2-dichloroethane and the corresponding capacitances, were calculated by using the modified Poisson-Boltzmann theory (version MPB4), which accounts for both the finite ion size and image forces. The two diffuse layers, one in water and the other in organic solvent, were assumed to be independent, ie they were associated only through an equal and opposite surface charge density. As compared with the Gouy-Chapman theory, the MPB theory provided a more reasonable description of the interface over the whole range of electrolyte concentrations and surface charge densities even in the system comprising the solvent of a low dielectric permittivity, such as 1,2-dichloroethane. By using the MPB theory, the interfacial potential difference and the inverse capacitance for both the systems studied were shown to split into two contributions, of which one is essentially independent of the electrolyte concentration and can be attributed to the presence of an ion-free inner (or compact) layer. The effect of the surface charge is to reduce the thickness of this layer, eg through the liquid phase interpenetration.