Mixed molybdenum-tungsten oxides of varying stoichiometry (Mo xW1-x-O3, 0 < x < 1) prepared by cathodic electrodeposition from aqueous peroxo-polymolybdotungstate solutions on indium tin oxide (ITO) coated glass substrates were evaluated by variable-angle spectroscopic ellipsometry (VASE) and transmission measurements from 200 to 1000 nm (1.24-6.2 eV). A Taue-Lorentz dispersion model was used to determine the real and imaginary components of the complex refractive index for Mo xW1-xO3 films as a function of Mo fraction. The optical band gaps were also estimated from Taue plots. The refractive index increased (2.07-2.20 at 800 nm) while the optical band gap decreased (3.38-2.95 eV) in a linear fashion for MoxW11-xO3 films with increasing Mo fraction. These trends correlate chiefly with Mo-doping-induced changes in film structure and grain size as supported by X-ray diffraction measurements.