Electroreduction of heterotetranuclear complexes (mu(4)-O)L(4)Cu(4-x)Ni(x)(H2O)(x)Cl-6 (x = 1-4, L = N,N-diethylnicotinamide) at a Pt electrode in dimethylsulfoxide leads to deposition of Cu-Ni alloys with codeposition of Cu(I) oxide, Ni(II) oxide, and Ni(II) hydroxide. The alloy deposition potential is invariant with complex stoichiometry. Alloy Ni composition, determined by x-ray diffraction (XRD), increases from 12% for x = 1 to 62% for x = 4. The microscopically rough, well-adhering, continuous films have a natural passivation layer formed by air oxidation that consists of Ni(OH)(2), NiO, Cu(OH)(2), and Cu2O. X-ray photoelectron spectroscopy confirmed the bulk film alloy compositions obtained by XRD. The data revealed complex deposit structures consisting of NiO, Ni(OH)(2), Cu2O, and Cu-Ni alloy giving a mass balance of the metals in the complexes. The Cu2O/Cu-0 ratio is close to unity for the deposit made from the Cu, complex and decreases to zero for the CuNi3 complex. In contrast only half of the Ni(II) centers are deposited as Ni-0 in the Cu-Ni alloy, the balance consisting of 37% NiO and 15% Ni(OH)(2). The constant percentage of Ni as Ni(OH)(2) in all deposits suggests that it arises from reduction of Ni coordinated water. Mass balance indicates O in Cu2O and NiO originates from the mu(4)-O. Smooth variations of alloy compositions, metal oxide/metal ratios, and film particle sizes indicate that all the electrode processes involve discrete molecules of the heteropolymetallic complex.