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 Brønsted 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 Brønsted acid sites were generated from H₂ on the Ru/Cs₃PW₁₂O₄₀ catalyst. The H₂-originated reversible Brønsted 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.