The deactivation of a diesel SCR (Selective Catalytic Reduction) catalyst by impregnation with CaSO4 was studied. The loss in activity after 2729 h of use on the engine was lower than when deactivated by impregnation. This was the case even if the amount of Ca in the catalyst was high after use on the engine. In the used catalyst a surface layer of CaSO4 was built up on the monolith during use. This causes lower deactivation than when CaSO4 was introduced in the pore system of the catalyst. The kinetics of the SCR reaction, the oxidation of NH3 to N-2 and two side reactions leading to N2O was used in the simulation. A coverage dependent heat of adsorption of NH3 was used. The heat decreased with the coverage and with the introduction of CaSO4 into the pores. After fitting the amounts of NO and N2O at the exit of the reactor against the temperature at the exit of the bed excellent agreements were obtained for the products N-2 and H2O as well for the reactant NH3 . This is the first time that the effect of CaSO4 poisoning of SCR catalysts and its effect on the kinetics is published.