The main objective of the present work was the study of the effect of hydrothermal pretreatment of lignocellulosic biomass (beech wood) on the product yields and bio-oil composition produced from biomass flash pyrolysis as well as from the catalytic upgrading of the biomass pyrolysis vapours. The hydrothermal pretreatment of lignocellulosic biomass was performed at a severity factor (Ro) of 3.55 leading to similar to 35 wt.% loss of solids, mainly due to solubilization and removal of hemicellulose. The production of sugars (mainly levoglucosan) was significantly increased by the use of the hydrothermally pretreated biomass instead of the untreated biomass in the non-catalytic flash pyrolysis experiments. On the other hand, the concentration of carboxylic acids, ketones and phenols was decreased in the bio-oil derived from the pretreated biomass. The catalysts tested in the upgrading of the biomass pyrolysis vapours were the strongly acidic zeolites H-ZSM-5 and silicalite (with very low number of acid sites) and the mildly acidic mesoporous aluminosilicate Al-MCM-41. The effect of catalysts on product yields and composition of bio-oil in the upgrading of pyrolysis vapours, was similar for both the pretreated and untreated biomass. The use of zeolite H-ZSM-5 decreased the total liquid yield (bio-oil) via decreasing the organic phase of bio-oil and increasing its water content, accompanied by increase of gases and moderate formation of coke on the catalyst. The zeolite silicalite and the Al-MCM-41 induced similar effects with those of H-ZSM-5 but to a less extent, except of the significantly higher coke that was deposited on Al-MCM-41. With regard to the composition of the bio-oil, all the catalysts and mostly the strongly acidic H-ZSM-5 zeolite reduced the oxygen content of the organic fraction, mainly by decreasing the concentration of acids, ketones and phenols in the untreated biomass pyrolysis oil or the concentration of sugars in the pretreated biomass pyrolysis oil. Aromatics and polycyclic aromatic hydrocarbons (PAHs) were significantly increased by the use of all catalysts, for both types of biomass feed. A substantial increase in the concentration of phenols was observed in the upgraded bio-oil derived by the hydrothermally pretreated biomass, using the less acidic silicalite and Al-MCM-41 catalysts. (C) 2011 Elsevier B.V. All rights reserved.