Here we report an electrochemical reduction-induced photocurrent enhancement up to an order of magnitude for monoclinic tungsten trioxide (WO 3 ) particulate photoanodes. Electrochemical impedance and photoelectrochemical measurements suggest that the improved performance is attributed to the increase in majority carrier concentration (from 1.5 × 10 18 to 2.5 × 10 21 cm −3 ). This results in higher conductivity and improved electron transport. Minority carrier extraction can be increased by reducing the WO 3 particle size from 200 nm to 50 and 30 nm. The larger solid-liquid interface area promotes the water oxidation rate. By balancing electron/hole extraction with photon absorption in an optimized 30 nm WO 3 particulate electrode, a record water oxidation photocurrent of 3.8 mA/cm 2 , under +1.36 V (vs. RHE) applied bias in 0.1 M Na 2 SO 4 solution at pH = 3.5 is achieved with 50 mW/cm 2 unfiltered Xe illumination. Photoelectrochemical water splitting is an efficient way of converting solar energy to chemical fuel by decomposing water into hydrogen and oxygen.1-9 Among inorganic materials that catalyze the photoelectrolysis of water, WO 3 with a bandgap of 2.6 eV is attractive due to its visible absorption and its photochemical stability in acid aqueous solution up to pH 5. [10][11][12][13] Since it was first reported as a potential photoanode for photoelectrochemical cells (PEC) 27-32 and enhancing WO 3 stability in neutral solution using surface coating. 27 Recently, Yat Li's group reported that a hydrogen treatment at 350• C of hydrothermally synthesized WO 3 films enhanced photoactivity and photostability of the WO 3 electrode, due to the incorporation of W 5+ donors and oxygen vacancies. 24 The W 5+ states are believed to be more resistant to photocorrosion by peroxo-intermediates formed during water oxidation and they improve electron transport in the WO 3 films. Herein, we demonstrate that a similar performance increase can be achieved by in-situ electrochemical reduction of WO 3 particle films at +0.17 V vs. RHE (−0.04 V vs. NHE) in aqueous electrolyte. The reduction increases the water oxidation photocurrent of WO 3 by over an order of magnitude, up to 3.8 mA/cm 2 for an optimized device. This is comparable to the highest observed water oxidation photocurrents for WO 3 films reported in the literature (>2.5 mA cm −2 , AM 1.5, E App = 1.0 V vs. NHE). 18,23,[33][34][35][36] Furthermore, this photocurrent enhancement is achieved without the utilization of sophisticated film geometry design 18 or water oxidation electrocatalysts. [27][28][29][30][31] The relative photocurrent enhancement is most pronounced for films composed of bulk WO 3 particles (enhancement factor of 14 fold), followed by films of 50 nm particles (10 fold) and 30 nm particles (2 fold). The reduction treatment also improves the stability of WO 3 against photocorrosion; after reduction, 50% of the photocurrent persists after 1 hr operation, compared to 10% for an untreated film. The enhancement is preserved in air, when films are at roo...