Molybdenum and tungsten oxides are of interest as semiconductors for the production of clean and sustainable energy. Here we show that synergistic effects arising from a combination of noncrystallinity and plasmonic resonance in mixed molybdenum/tungsten oxides can lead to improved efficiency for the photoelectrochemical (PEC) splitting of water. The quasi-binary Mo/W oxides were synthesized solvothermally on a gram scale. Size, structure, morphology, and electronic properties of the as-prepared microspheres were characterized by scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), Raman, optical absorption (UV-vis), and X-ray photoelectron spectroscopy (XPS). Molybdenum oxide benefits from W-substitution and the concomitant metal reduction. The increased number of charge carriers leads to higher photocurrents for Mo0.5W0.5O2.1 (5.25 mA cm(-2)), the most reduced phase compared to Mo0.89W0.11O2.7 (1.75 mA cm(-2)). Long-term photocurrent stability tests (2000 s) under photoillumination confirmed the chemical stability of Mo/W oxides under sunlight. The improved PEC performance is attributed to the synergistic effect of increased charge carrier concentration due to metal reduction, suppressing the formation of crystalline metallic oxides through disorder, and tuning the absorption in the visible and near-IR range by the formation of W5+ and Mo5+ sites.