Water solutions of glucono-delta-lactone and glucono-gamma-lactone in equilibrium with gluconic acid were hydrogenated with molecular hydrogen in the presence of a commercial carbon-supported ruthenium catalyst (5% Ru/C, Engelhard Escat 40). Reactions were conducted batchwise on 20 wt% solutions under 100 bar pressure in the temperature range 80-140 degrees C. Reaction rates were unexpectedly high at moderate temperatures (e.g., 0.58 mol h(-1) g(Ru)(-1) at 100 degrees C) because lactones were probably the reactive species. The selectivity to sorbitol at total conversion was larger than 99% at temperatures lower than 100 degrees C. Kinetic study was done at different temperatures, pressures and concentrations. Modelling of the reaction kinetics showed that the reaction followed a rate law corresponding either to the Langmuir-Hinshelwood mechanism without H-2 dissociation, or to the Horiuti-Polanyi mechanism.