In the development of new processes that provide "green energy", supercritical water (SCW) has emerged as a powerful reaction medium to convert biomass into combustible gases such as hydrogen or methane. Due to typical SCW catalytic process conditions (400 degrees C, 25 MPa), in situ characterization of materials and catalysts used in selective biomass conversion is difficult, and accordingly, there is limited knowledge about catalyst structure and reaction pathways under these conditions. Particularly, catalyst-poisoning mechanism by sulfur, a major obstacle in catalytic biomass conversion, needs to be understood in order to design sulfur-resistant catalysts and regeneration procedures. We followed the dynamic structural changes of a Ru catalyst during the conversion of biomass model compounds (methanol and ethanol) to methane in supercritical water in a continuous flow reactor. In situ X-ray absorption spectroscopy (XAS) showed that the catalyst is being activated by the organic compounds at low temperature without a change in particle size over 8 h of operation. Combining XAS with isotope labeling and electronic structure calculations, we demonstrated that sulfur poisoning proceeds via irreversible adsorption of S2- with a surface coverage of about 40% instead of bulk sulfidation. The adsorption of sulfur significantly changes the nature and abundance of hydrocarbon adsorbates - the precursors for methane formation - on the catalyst's surface. This affects both the activity and selectivity of the catalyst for the methanation reaction. These results provide an incentive for designing sulfur-resistant catalysts or effective regeneration procedures. (C) 2013 Elsevier Inc. All rights reserved.