Bio-oil (product liquids from fast pyrolysis of biomass) is a complex mixture of oxygenates derived from the thermal breakdown of the biopolymers in biomass. In the case of lignocellulosic biomass, the structures of three major components, cellulose, hemicellulose and lignin, are well-represented by the bio-oil components. To study the chemical mechanisms of catalytic hydroprocessing of bio-oil, three model compounds were chosen to represent those components. Guaiacol represents the large number of mono- and dimethoxy phenols found in bio-oil derived from soft- or hardwood, respectively. Furfural represents a major pyrolysis product group from cellulosics. Acetic acid is a major product from biomass pyrolysis, derived from the hernicellulose, which has important impacts on the further processing of the bio-oil because of its acidic character. These three compounds were processed using a palladium or ruthenium catalyst over a temperature range from 150 to 300 degrees C. The batch reactor was sampled during each test over a period of 4 h. The samples were analyzed by gas chromatography with both a mass selective detector and a flame ionization detector. The products were determined, and the reaction pathways for their formation are suggested on the basis of these results. Both temperature and catalyst metal have significant effects on the product composition.