Biomass can be pyrolytically converted in high yields (similar to 70 wt.%) into vapors (or oils when condensed) composed mainly of oxygenated organic compounds. Using a fixed-bed microreactor interfaced with a molecular beam mass spectrometer (MBMS), we have been studying the catalytic steam reforming of model oxygen-containing compounds present in biomass pyrolysis vapors. This MBMS sampling system is unique in its rapid, real-time, and universal detection of gaseous and condensible products. In this paper, we present results for steam reforming of acetic acid (HAc) and hydroxyacetaldehyde (HAA), two major products derived from the pyrolysis of carbohydrates in biomass. We propose mechanisms to couple the thermal decomposition and steam reforming reactions of these compounds. Both HAc and HAA undergo rapid thermal decomposition; complete steam reforming of these two model compounds can be achieved with commercial Ni-based catalysts. HAc forms coke on the catalyst surface, which is subsequently gasified by steam. The proposed mechanism for this coke formation involves an adsorbed acetate species that decarboxylates to form the coke precursor, (CH1-3)(abs), and also ketene, a dehydration product of HAc, that decomposes to form (CH1,2)(abs). The reforming of HAA by steam does not involve any detectable intermediate and proceeds smoothly to a complete breakdown to CO and H-2 on the catalyst surface.