Semiconducting CdSe thin films were synthesized by cathodic electrodeposition from an aqueous acid solution. Alloy formation occurs when the growth rate is limited by selenium ion mass transport and when the cadmium is underpotentially deposited. The composition and the physical properties of the asgrown layers are very sensitive to the selenium concentration of the solution and to the applied potential. CdSe films always have a zinc blende structure (instead of the normal wurtzite structure) with a strong (111) preferred orientation. It is shown that the best characteristics (stoichiometry, crystallinity, band gap width, etc.) are obtained with low selenium concentrations and in a narrow potential range. By using these appropriate preparation conditions, it will be possible to reduce the chemical and physical post-treatments generally applied to these kinds of semiconducting thin films in order to increase their solar energy conversion performances in solid or liquid junction cells.