CdSe and mixed CdSe and CdTe semiconducting thin films, prepared by cathodic electrodeposition from an acid sulphate solution, containing selenium and tellurium oxides in Various amounts, were submitted to a to a thermal treatment at temperatures ranging between 400 and 520 °C. The crystal structure, composition, band-gap width and photoelectrochemical response of the annealed materials was investigated. It was found that all materials, rich in selenium, change their structure from cubic (zinc blende) to hexagonal (wurtzite), when annealed within the above mentioned region of temperatures. In many cases, an improvement of their semiconducting properties has been also confirmed.