The electrodeposition of Mo x Re1−x O y films (0.6≤ x ≤1) on indium-tin oxide (ITO) coated glass substrates from acidic peroxo-polymolybdo-perrhenate solutions is described. Trends in film growth were established as a function of potential from +0.4V to −0.7V vs Ag/AgCl by analyzing the composition and stoichiometry of the deposit using inductively coupled plasma mass spectrometry (ICPMS) and X-ray photoelectron spectroscopy (XPS). These experiments show that the concentration of rhenium increases linearly with the deposition potential and that the deposits are mixed-valent containing up to five different metal oxidation states (i.e., MoIV, MoV, MoVI, Re0, ReIV). Electroanalytical techniques were used to explore the deposition mechanism, including chronocoulometry, cyclic voltammetry, spectroelectrochemistry, and electrochemical quartz crystal nanogravimetry (EQCN). At potentials positive to −0.26V, perrhenate (ReVIIO4 −) behaves as a redox mediator to accelerate the deposition of a mixed-valent molybdenum oxide, but at more negative potentials mixed molybdenum–rhenium oxides are produced.