Electrodeposition mechanisms of Se, Cu and Cu 2 - x Se thin films were studied by means of the electrochemical quartz crystal microbalance (EQCM). The electrodeposition of Se was found to be faster at higher overpotentials, though with lower apparent efficiency. The formation of Cu 2 - x Se from a thiocyanate bath was confirmed to proceed via the induced co-deposition mechanism at more positive potentials than where Cu + alone is reduced. The M/z value corresponding to Cu 2 - x Se formation first decreased gradually during the cyclic voltammogram, indicating that the previously formed Se film was converted into Cu 2 - x Se. After the Se film was consumed, the M/z value settled to a constant value, which corresponded to the formation of Cu 2 - x Se from Cu + and Se 4 + ions. The M/z value was constant over a wide potential range. In contrast, when the Cu 2 - x Se films were deposited at constant potentials, the M/z value was found to be dependent on the deposition potential.