The electrodeposition of Ni-Mo alloy using a series of aqueous acidic citrate plating baths (pH 5.0) were examined from the standpoint of the Ni(II)- and Mo(VI)-species present in the solutions. The distribution of the species was calculated using a set of the formation constants for polymolybdates and citrate complexes, optimized by a factor analysis of optical spectra. Through the design of the bath compositions such that the dominant Ni(II)- and Mo(VI)-species are both 1: 1 complexes with citrate, lustrous and uniform Ni-Mo layers having almost constant Ni content were found to be plated over a wide range of Ni(II)/Mo(VI) analytical concentration ratios, although the current efficiency decreased with decreasing Ni(II) concentration. This indicates the validity and importance of the bath design using these constants. The deposition phenomena were briefly discussed in terms of the mechanisms hitherto reported. (C) 2004 The Electrochemical Society.