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Solvent extracts from spent commercial naphtha and gas oil hydrotreating catalysts were examined in order to elucidate the structure of adsorbed poisons. Amides were identified in the extracts and made up 20-30% of the basic compounds. The oxygen content of the extracts was as high as 28% by weight. Some of this oxygen may be associated with metal complexes or present as inorganic material. Infrared spectroscopic analysis revealed very strong absorptions due to carbonyls, suggesting that compounds such as ketones and carboxylic acids were present. The extracted compounds consisted of saturated (paraffinic and naphthenic) structures and aromatic groups, with >50% boiling at +343 °C. These results suggest polymerization of species on the surface of the catalyst, rather than condensation of aromatic structures to form coke.