The importance of hydrodeoxygenation (HDO) which occurs during hydroprocessing depends on the origin of feeds. HDO plays a minor role in the case of the conventional feeds, whereas for the feeds derived from coal, oil shale, and, particularly from the biomass, its role can be rather crucial. The mechanism of HDO was established using a wide range of model compounds. Complexities in the HDO kinetics have been attributed to the self-inhibiting effects of the O-containing compounds as well as inhibiting and poisoning effects of the S- and N-containing compounds present in the feeds. This is a cause for some uncertainties in establishing the order of the relative HDO reactivities of the O-containing compounds and/or groups of the compounds as well as relative rates of the removal of S, O and N. Complexities arise particularly for real feeds. This is supported by deviations from the established order such as HDS>HDO>HDN. The cases for which the overall HDN was greater than HDO were also observed. In this case, distribution of the O- and N-containing compounds in the feed and the type of catalyst are of a primary importance. HDO is the main reaction which occurs during hydroprocessing of the bio-feeds. The current research activities in HDO are predominantly in this area. Apparently, more stable catalysts are needed to make production of the commercial fuels from the bio-feeds more attractive. (C) 2000 Elsevier Science B.V. All rights reserved.