Abstract: The cathodic deposition mechanism of CdSe films on Ti substrates was investigated using cyclic voltammetry and in-situ photocurrent mesurements. XPS analysis showed that CdSe film composition depends on deposition potentials, deposition time and concentration of H_2SeO_3 in the bath. It was found that in 0.1 mol·L~(-1) CdSO_4+4 mmol·L~(-1) H_2SeO_3+0.2 mol·L~(-1) H_2SO_4, the deposits formed at potentials more positive than -0.50 V are rich in Se, alternatively films rich in Cd are obtained at potentials more negative than -0.70 V. The Se/Cd ratio of CdSe layers deposited at -0.69 V approach to 1.1, but the content of Se in the deposits increases with increasing H_2SeO_3 concentration. The results showed that the electrodeposition of CdSe follows two different mechanisms corresponding to different deposition conditions. Modification of stoichiometry in CdSe films was also discussed.