Molybdenum-selenium oxides were electrochemically deposited onto indium-tin oxide (ITO) coated glass substrates from aqueous solutions containing molybdate (MoVI O42 -), selenate (SeIV O32 -), and dimeric and tetrameric peroxo-polymolybdate (i.e., [Mo2O3(O2)4(H2O)2]2-, [Mo4O9(O2)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, Mo3O8, co-deposits with an insulating phase of Se0. At more negative potentials, mixed molybdenum-selenium oxides (MoxSe1-xOy, 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.