The aim of this work is to analyze the behaviour of a thin layer of semiconducting CdS, used as photosensible anode in a photoelectrolyitical cell, when the same electrode is covered by a thin layer of a non-conducting polymer. Such a structure should permit a protection against the photocorrosion effects, that, as well known, are considerable for the Cadmium Sulfide and, generally, for all the semiconductors with a small value of the interband gap. This procedure ought to allow a long term stability of the electrodes. With this purpouse it has been measured the rate at which the photocurrent, exhibited by the dispositives, decreases in time as a function of the polymer thickness. The choose of the Formvar (polyvinylformale) as the polymer, was suggested by the large use made, in former times, by the same authors, of this material in junction devices, where the electric charge conduction was by tunnel effect.