The deactivation mechanism of the gas-phase synthesis of o-hydroxyacetophenone (o-HAP) via acylation of phenol with acetic acid was studied on A1-MCM-41 and zeolites HY, HBeta and HZSM-5. The o-HAP yield remained constant with time-on-stream on HZSM-5 but drastically decreased on the other samples because of coke formation. The origin and nature of coke precursor species were studied by employing as reactants, the key reaction intermediates and products involved in the phenol acylation reaction network. It was found that coke precursors are formed from consecutive condensation reactions and not from any ketene formation by phenyl acetate decomposition. Specifically, it is proposed that o-acetoxyacetophenone (o-AXAP), which is formed by reaction between o-HAP and acetic acid, is the key intermediate specie responsible for the formation of carbonaceous deposits and consequently, for the activity decay observed on A1-MCM-41, HY and HBeta samples. The narrow pore size structure of zeolite HZSM-5 would hinder the formation of bulky o-AXAP, thereby, decreasing drastically the formation of coke. (c) 2005 Elsevier B.V. All rights reserved.