Molybdenum-selenium oxides were electrochemically deposited onto indium-tin oxide (ITO) coated glass substrates from aqueous solutions containing molybdate (Mo(VI)O(4)(2-)), selenate (Se(IV)O(3)(2-)), and dimeric and tetrameric peroxo-polymolybdate (i.e., [Mo(2)O(3)(O(2))(4)(H(2)O)(2)](2-), [Mo(4)O(9)(O(2))(4)](2-)) anions. Electrodeposition mechanisms were elucidated using chronocoulometry, cyclic voltammetry, spectroelectrochemistry, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical quartz crystal nanogravimetry (EQCN). At relatively positive deposition potentials from -0.1 V to -0.4 V (versus Ag/AgCl) a substoichiometric molybdenum oxide phase, Mo(3)O(8), co-deposits with an insulating phase of Sea. At more negative potentials, mixed molybdenum-selenium oxides (Mo(x)Se(1-x)O(y), 0 < x < 0.4) are deposited. Competing side reactions involving hydrogen and hydrogen selenide influence the structure, composition, and morphology of the mixed molybdenum-selenium oxide deposits. (c) 2009 Elsevier B.V. All rights reserved.