The hydrotreatment of various oxygenated groups (ketonic, carboxylic, methoxyphenol) present in bio-oils in the presence of CoMo and NiMo catalysts was studied in a batch reactor using a mixture of model compounds mimicking the real feed. The influence of potential poisons or inhibitors of the reactions (water, ammonia and hydrogen sulfide) was determined. High quantities of water had only a very slight inhibiting effect on the reactions. Ammonia strongly inhibited the conversion of carboxylic esters and the removal of the methoxy group of guaiacol, but, surprisingly, the hydrogenation of the ketonic group was not affected. Hydrogen sulfide depressed the activity of the NiMo catalyst for the conversion of the ketonic group but not that of the CoMo catalyst. It had an enhancing effect on the conversion of the carboxylic ester group and no effect on the removal of the methoxy group of guaiacol. The evolution of activities and selectivities as a function of the concentration of potential inhibitors provided an indication of the catalytic sites responsible for the various reactions. The absence of the influence of ammonia on the hydrogenation of the ketonic group was interpreted as resulting from the participation of nucleophilic sites and hydridic species in the reaction mechanism. Carboxylic esters seem to react on electrophilic sites. Bronsted acids were thought to be responsible for decarboxylation, while uncoordinated metal atoms and sulfhydryl groups could be responsible for the hydrogenation of carboxylic groups. On the other hand, the surface of the alumina support catalyzes the hydrolysis of carboxylic esters into acids. The demethylation of guaiacol occurs for a large part on the Lewis acid-base sites of the gamma-alumina support. The use of hydrogen sulfide and ammonia shows a high potential for controlling the selectivity of reactions occurring in bio-oils hydrotreatment. The present results give hope that the deoxygenation of carboxylic groups could be selectively performed through decarboxylation thanks to catalyst selection and control of the hydrogen sulfide pressure. Ketonic groups and aldehydic groups could be selectively eliminated from complex feeds by applying a pressure of ammonia which would inhibit all reactions but hydrogenation.