Cellulose and inulin, two important biomasses, can be transformed to polyols using bifunctional catalysts combining acid sites for hydrolysis and metal nanoparticles for hydrogenation. Here, we report that Ru nanoparticles loaded on a Keggin-type polyoxometalate, i.e., Cs3PW12O40, without intrinsic Br?nsted acidity exhibit superior catalytic performances for the transformation of inulin and cellulose into hexitols in water in the presence of H2. We demonstrated that new Br?nsted acid sites were generated from H2 on the Ru/Cs3PW12O40 catalyst. The H2-originated reversible Br?nsted acid sites were robust during the transformation of biomass under hydrothermal conditions. We further found that the mean size of Ru nanoparticles determined the product selectivity in the conversion of inulin under H2. The catalyst with larger Ru nanoparticles favoured the formation of fructose, the hydrolysis product, while the major products were hexitols over the catalyst with a smaller Ru particle size. We clarified that, as compared to that of inulin hydrolysis, the rate of fructose hydrogenation increased more rapidly upon decreasing the Ru particle size. ? the Partner Organisations 2014.