Cadmium sulphide is a semiconductor material of great interest because it has the potential to play an important part in practical solar energy conversion devices such as Cu2S/CdS heterojunction cells [1, 2] and as electrodes in photoelectrochemical cells [3]. CdS shows two polymorphic cases; one is hexagonal (wurtzite) and the other cubic (zincblende). For its application in photovoltaic cells it is necessary that the growth phase should be the hexagonal one. A variety of techniques for the formation of this material have been described: evaporation [4], sputtering, spray, chemical bath deposition [5] and electrodeposition in aqueous and non-aqueous media [6, 7]. We 'have studied the electrochemical deposition of CdS thin films on different substrates in aqueous solutions [8]. In this work, the structural, optical and morphology of CdS films on aluminium substrates obtained by this method has been studied. The CdS films were grown by electrodeposition in a solution containing 2 x 10 3M C d S O 4 , 0.1M Na2S203 adjusting the pH to 2.3 with H2SO4. Under these conditions, the SzO32is decomposed to colloidal sulphur, which is reduced to sulphide ions which, together with Cd 2 present in the interface, form a CdS film on the electrode. Fig. 1 shows the voltammogram corresponding to the formation of CdS on an aluminium electrode at pH = 2.3, the cathodic current appearing between 800 and 1000 mV with respect to SCE (standard calomel electrode). When 1 0 0 0 mV with respect to SCE was applied for a sufficiently long time, the electrode became covered with a yellow uniform translucent rather thick film. For shorter electrolysis times, thin nonadherent CdS films were formed. Therefore, long electrolysis times were used (6 h).