The equilibrium constants of the formation of isopolymolybdates HrMo7O24(6-x)- (x = 0-2), heteropolymolybdate NiMo6O24H64- and Mo(VI)-citrate complexes H-r(MoO4)(2)Cit((7-r)-) and H(r)MoO(4)Cit((5-r)-) (Cit(3-) = C6H5O73-) were determined from factor analysis of Raman spectra and visible absorption spectra to calculate the chemical species present in acidic Ni-Mo alloy plating baths. On the addition of citrate to a Ni(II)-Mo(VI) bath, (i) the polymolybdates were decomposed by the formation of H-r(MoO4)(2)Cit((7-r)-) complex, and then (ii) H(r)MoO(4)Cit((5-r)-) and NiCit(-) complexes formed. This complex formation was necessary for the alloy deposition, but an excess addition of citrate, which results in the formation of NiCit(2)(4-) complex, lowered the current efficiency. The deposition behavior of Ni-Mo alloy from Ni(II)-Mo(VI)-citrate baths was compared with that of pure Ni from Ni(II)-citrate baths, in which concentrations of Ni(II)-species are identical to those in the corresponding Ni(II)-Mo(VI)-citrate baths. The alloy deposition behavior is strongly governed by the ease of Ni deposition, indicating that the electrodeposited nickel induces the deposition of molybdenum.