The induced electrodeposition of Ni-Mo alloy was studied to elucidate the mechanism involved in terms of the chemical species present in plating baths, which affects the behavior of codeposition. To determine the chemical species in acidic Ni(II)-Mo(VI)-citrate aqueous baths with pH 5, factor analysis of visible absorption spectra was employed as were previous results by extended X-ray absorption fine structure (EXAFS) and anomalous X-ray scattering. In citrate-free acidic Ni(II)-Mo(VI) baths (pH 5) from which codeposition of molybdenum metal is impossible, the ions form a large Ni 2+ -MoO 4 2- cluster, NiMo 6 O 24 H 6 4 - . To deposit Ni-Mo alloy, a sufficient amount of citrate must be added to break down the Ni 2+ -MoO 4 2- cluster so that Ni 2+ and citrate ions form complexes, NiCit - and NiCit 2 4- (Cit 3- = C 6 H 5 O 7 3- ). An insufficient addition of citrate, however, restrains the complexing between Ni 2+ and citrate, since citrate is preferentially consumed for complexing with MoO 4 2- , and codeposition does not occur. Profiles of the molar absorption coefficient for aquated species Ni 2+ , NiCit - , and NiCit 2 4- , were obtained by factor analysis.