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Gas phase toluene hydrogenation is investigated over Pt/Al 2 O 3 catalyst with temperature ranging from 75 to 125°C and at atmospheric pressure. Strong activity variations are observed during long duration experiments. These variations are thoroughly investigated and a mechanistic model is proposed with dynamic adsorption activity of the reactants, used to explain the decrease in catalyst activity. This model considers competitive adsorption behaviour of the reactants and dissociative adsorption of hydrogen. Such a model can also be used to explain the strong metal-support interaction (SMSI) effect induced by the catalyst support. The decrease in activity after temperature maxima as previously observed can also be addressed by the approach presented. A comparison of activity variation at different residence times i.e. 20-50 kg cat ÁsÁmol -1 and different hydrogen and toluene partial pressures is also simulated.