FTIR and microcalorimetric analysis of adsorbed pyridine has been employed for the characterisation of a set of Fe-MFI zeolite catalysts with different Si/Al and Si/Fe ratios, used for the gas phase hydroxylation of benzene to phenol by N2O. Catalysts behaviour in terms of activity and deactivaton rate is discussed in relation to the nature, concentration and strength of their surface acid sites. Surface acidity, though not involved directly in the hydroxylation reaction, plays a major role in determining catalyst lifetime. Coke formation, leading to catalyst deactivation, is triggered by further undesired reactions of phenol and is favoured by the strong adsorption of the latter on Lewis acid sites, restraining its diffusion out of the zeolite pores. (C) 2003 Elsevier B.V. All rights reserved.