Summary The investigation of single catalyst pellet deactivation is studied. The model system the hydrogenation of ethylene on the Pt/alumina catalyst (narrow region of Pt inside the pellet) in the presence of the catalyst poison thiophene is considered. It was found that the deactivation mechanism consists of two consecutive steps: reversible catalyst poison precursor adsorption and irreversible surface reaction. The reversibility of the first step was inferred experimentally. To model the temperature gradient in the catalyst pellet it was necessary to include the thermal conductivities of both pellet and thermocouple.