Ortho-xylene isomerisation on H-MFI zeolite at temperatures between 393 and 573 K was studied in an infrared reactor-cell. 2D correlation analysis of the spectra recorded during the experiment allowed the detection of traces of coke on the catalyst, and the determination of the OH groups specifically perturbed by the coke molecules. Adsorption of collidine and pyridine on fresh and coked samples was used to determine the location of these OH groups in the microporous structure. It appears that coke is not perturbing Bronsted acidity (bridged OH) under these conditions on the MFI zeolite. It is mainly located on silanol groups; these silanols are not located on the external surface of the crystallites, but at defects located inside the pore system.