Structure sensitivity in hydrogenation of arabinose and galactose was studied over supported ruthenium catalysts with the cluster size ranging from 1 to 8 nm. Ruthenium on carbon supports with different ruthenium particle sizes were prepared by incipient wetness impregnation and colloidal method. Catalytic activity was seen to have almost no dependence on the substrate being, however, sensitive to the cluster size. The highest turnover frequency (TOF) was obtained with a catalyst having average ruthenium particle size of ca. 3 nm. A mathematic model incorporating cluster size dependent rate constants was used to quantitatively describe experimental data on TOF.