The behaviour of HZSM-5 zeolite in the upgrading of a wood pyrolysis oil produced in the ENEL fast-pyrolysis plant located in Bastardo, Italy, was studied in repeated upgrading±regenerating cycles. The HZSM-5 zeolite performs a catalytic activity by its acidic sites that, through a carbonium ion mechanism, promote deoxygenation, decarboxylation and decarbonylation of the oil constituents, as well as cracking, oligomerisation, alkylation, isomerisation, cyclisation and aromatisation. As a consequence of the catalytic process, coke and tar were also obtained as undesirable by-products. The continued regeneration of the zeolite, consisting of removal of the coke deposits by air at 5008C, reduced the effectiveness of the catalyst in converting biomass pyrolysis oils to an aromatic product, until an irreversible deactivation was observed. By the analysis conducted on the catalyst it was possible to assess that the loss of activity is mainly connected to the disappearance of a signi®cant amount of acidic sites, mainly the stronger ones, due to the thermal cycling to which the catalyst was submitted. Even if the regeneration was conducted at 5008C, localised raisings of temperature above 5008C due to the combustion of coke may have caused dehydroxylation of the Brùnsted acid sites that predominate in zeolites activated at 5008C with formation of Lewis acid sites. Thus, the active acid sites in the upgrading reactions are presumed to be preferentially Brùnsted acid sites, which were gradually deactivated by the repeated regeneration treatments.