A novel and convenient method of monitoring the internal surface area of porous silicon layers grown on either n-type or p-type silicon is described. The method involves measurement of the interfacial capacitance under conditions where electrons accumulate at the electrode surface. The technique has been used to study the growth and leaching of porous silicon layers in hydrofluoric acid/ethanol. The changes in the interfacial capacitance have been related to the development of photoluminescence.