Hydrodeoxygenation (HDO) of pyrolysis oil is an upgrading step that allows further coprocessing of the oil product in (laboratory-scale) standard refinery units to produce advanced biofuels. During HDO, desired hydrotreating reactions are in competition with polymerization reactions that can lead to unwanted product properties. To suppress this polymerization, a low-temperature HDO step, referred to as stabilization, is typically used. Small batch autoclaves have been used to study at near isothermal conditions the competition between hydrotreating and polymerization reactions. Although fast polymerization reactions take place above 200 degrees C, hydrogen consumption was already observed for temperatures as low as 80 degrees C. Hydrogen consumption increased with temperature and reaction time; however, when the end temperature exceeded 250 degrees C, hydrogen consumption achieved a plateau. This was thought to be caused by the occurrence of fast polymerization reactions and the refractivity of the products to further hydrotreating reactions. The effect of the gas-liquid mass transfer was evaluated by using different stirring speeds. The results of these experiments (carried out at 300 degrees C) showed that in the first 5 min of HDO, gas-liquid mass transfer appears to be limiting the overall rate of hydrotreating reactions, leading to undesired polymerization reactions and product deterioration. Afterward, intraparticle mass transfer/kinetics seems to be governing the hydrogen consumption rate. Estimations on the degree of utilization (effectiveness factor) for industrially sized catalysts show that this is expected to be much lower than 1, at least, in the early stage of HDO (first 30 min). Catalyst particle size should, thus, be carefully considered when designing industrial processes not only to minimize reactor volume but also to improve the ratio of hydrotreating to polymerization reactions. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 3160-3170, 2011