The use of homogeneous ruthenium catalysts to hydrogenate the water-soluble fraction of pyrolysis oil is reported. Pyrolysis oil, which is obtained by fast pyrolysis of lignocellulosic biomass at 450-600 degrees C, contains significant amounts of aldehydes and ketones (e.g. 1-hydroxy-2-propanone (1) and 1-hydroxy-2-ethanal (2)), that are known to have a negative effect on the product properties (a.o. storage stability) of pyrolysis oil. The hydrogenation experiments were performed at mild conditions (40 bar, 90 degrees C using a biphasic system (water/toluene) and RuCl2(PPh3)(3) as the toluene soluble catalyst. Significant reductions in the amounts of (1) and (2) were observed, demonstrating the potential of homogeneous Ru-catalysts to upgrade pyrolysis oils. Model studies showed that (1) and (2) are selectively hydrogenated to 1,2-propanediol and 1,2-ethanediol, respectively. The influence of the temperature (50-90 degrees C), pressure (20-40 bar) and initial substrate concentration on the reaction rates were investigated. For (2), the reaction was shown to be first order in substrate and zero order in hydrogen. An overall kinetic model for the hydrogenation of (2) was developed including the rate of active catalyst formation and the kinetic parameters were determined. (c) 2006 Elsevier B.V. All rights reserved.