Monoclinic WO3 nanocrystals were synthesized by a hydrothermal reaction and post calcination. Their particle sizes were varied from 30 nm to 500 nm by changing calcination temperature from 500 degrees C to 800 degrees C. Photooxidation of water was studied in particulate suspension (PS) system and photoelectrochernical (PEC) film system. For PS system, WO3 nanocrystals were suspended in 50 mM AgNO3 solution to measure 02 evolution rate. For PEC system, WO3 films were fabricated by doctor blade method using synthesized nanocrystals. Photocurrent density was measured at AM 1.5 G (1 sun) solar condition in 0.5 M 2SO4. In PS system, the sample calcined at the highest temperature generated the largest amount of oxygen, whereas in PEC system the sample calcined at 600 degrees C showed the maximum photocurrent. The two systems also showed opposite response to deposition of the Pt co-catalyst. These different behaviors were attributed to different mechanisms of charge separation in the two systems. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.