An industrial FCC catalyst has been exposed in a MAT reactor to typical heavy fractions to obtain samples with different coke contents. These samples were investigated in the TAP reactor with toluene as probe molecule. A mathematical model has been developed that allows estimating diffusion coefficients in the meso- and micropores as well as the sorption parameters. Modeling of the TAP data provides insight into the deactivation mechanism due to coke deposition. At low coke content (<2.6 wt%), the enthalpy for toluene adsorption drops rapidly with increasing coke content indicating preferential blocking of the strongest acid sites. At higher coke contents (>2.6 wt%), blocking of the micropores occurs. The model describes the data adequately with physical meaningful parameters. (C) 2004 Elsevier Ltd. All rights reserved.