This study examined the dependence of Ru particle sizes upon the activity and selectivity for aqueous phase reforming reactions of ethanol and acetic acid over Ru/TiO2 at 473 K. For ethanol, irrespective of the Ru particle size, the 1:1 ratio of CH4 and CO2 was formed at the initial stage, indicating the absence of complete reforming with water. At the later stage, unfavorable methanation of CO2 by H-2 was observed over larger Ru particle catalysts. That methanation was suppressed completely over smaller Ru particle catalysts. In the case of acetic acid reforming over smaller Ru particle catalysts, a 2:1 ratio of H-2 and CO2 was formed constantly, indicating the operation of complete reforming reaction with water. Positively charged small Ru particles were responsible for the efficient activation of acetic acid and water to form H-2 and CO2 selectively. (C) 2013 Elsevier B.V. All rights reserved.