The chemistry of a commercial alkaline cyanide electroplating bath used for the deposition of a Cu-Zn-Sn alloy has been investigated. The voltammetry, the deposit composition and the morphology have been investigated as a function of the concentrations of the three metal ions, Cu(i), Zn(ii) and Sn(iv) as well as the concentrations of cyanide, hydroxide, carbonate and 'Copper Glo' additive. It is shown that all components combine to control the bath performance although the trends in alloy composition and quality can be predicted from the known chemistry of the bath. It is also concluded that the deposition of tin has an important role in initiating the growth of alloy layers.