CdTe, CdSe and CdSexTe1−x semiconducting compounds were prepared by cathodic electrodeposition from an acid sulphate solution containing TeO2 and H2SeO3 in various amounts. The composition of the layers, their crystal structure, morphology, band-gap width and their photoresponse in a photoelectrochemical (PEC) cell were investigated as functions of the electrolyte composition and the deposition potential. It is shown that smooth crystallized CdSexTe1−x thin films can be obtained only for small concentrations of H2SeO3 in the bath and in a restricted potential range corresponding to the under-potential deposition of cadmium.