Tungsten(VI) oxide (WO3) powders of monoclinic crystal structure with high crystallinity have been synthesized by hydrothermal treatment (HTT), at 523 or 573 K, of aqueous tungstic acid (H2WO4) solutions prepared from sodium tungstate by ion-exchange (IE) with a proton-type resin. HTT and/or calcination of home-made and commercial H2WO4 solids of tungstite structure also produced WO3. These WO3 powders with various physical properties were used as photocatalysts for evolution of oxygen (O2) from an aqueous silver sulfate solution, and the correlation between their physical properties and photocatalytic activity was examined. WO3 powders of high crystallinity, e.g., IE-HTT-WO3 synthesized at 573 K, gave the highest O2 yield, suggesting that the crystallinity of WO3 is a key factor in its high activity for this reaction system. Calcination at <1073 K improved the crystallinity of some WO3 powders, resulting in increases in their activities; however, calcination at a higher temperature reduced the activity, presumably due to formation of oxygen vacancies acting as recombination centers.