A rotating electrochemical quartz crystal microbalance is designed and used to study the electrodeposition of copper-selenium compounds onto gold from an aqueous solution of CuSO4 and H2SeO3. Thin films were prepared over a range of potential and pre cursor concentrations in the deposition bath. The composition of the films was determined in situ by simultaneous measurements of the quantity of charge and gains in the mass. It is shown that over a 200 mV potential range, the composition of the deposit is controlled by hydrodynamic conditions and the electrolyte composition. Deposit morphology is also discussed on the basis of the recorded changes in the mass scanning electron microscopy observations.