H-2 with ultra-low CO concentration was produced via photocatalytic reforming of methanol on Au/TiO2 catalyst. The rate of H-2 production is greatly increased when the gold particle size is reduced from 10 to smaller than 3 nm. The concentration of CO in H-2 decreases with reducing the gold particle size of the catalyst. It is suggested that the by-product CO is mostly produced via decomposition of the intermediate formic acid species derived from methanol. The smaller gold particles possibly switch the HCOOH decomposition reaction mainly to H-2 and CO2 products while suppress the CO and H2O products. In addition, some CO may be oxidized to CO2 by photogenerated oxidizing species at the perimeter interface between the small gold particles and TiO2 under photocatalytic condition. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.