Cu-Zn alloys were deposited under potentiostatic conditions from cyanide baths with varied Cu+/Zn2+ concentration ratio but with the same free CN- concentration. The Zn content of the deposits showed the maximum at around -1.4 V vs. Ag/AgCl as a function of the applied potential. The deposits obtained at -1.4 V vs. Ag/AgCl had the composition near CuZn, and the beta'-brass superlattice whose structure is the CsCl type, was independent of the Cu+/Zn2+ concentration ratio of the baths. In the absence of Cut in the bath, elemental Zn was deposited only at potentials negative to -1.5 V vs. Ag/AgCl and the Zn electrode was dissolved in the bath at -1.4 V vs. Ag/AgCl. This shows that Zn was codeposited with Cu forming the beta'-brass deposits in the underpotential deposition region. The beta'-brass deposits were formed through the accumulative underpotential deposition of Zn onto the electrodeposited Cu atom. The underpotential deposition of Zn onto Cu was confirmed from the cyclic voltammograms of a Cu electrode.