The competition between diffusion and first-order irreversible reaction in poisoned catalysts is revisited. Two cases are considered for isothermal slab catalysts: uniform and shell-progressive (or pore-mouth) poisoning. Analytical concentration profiles are derived, and the implications on catalyst performance are evaluated in different regimes (chemical- or diffusion-controlled) for different levels of poisoning. It was found that depending on the poisoning mechanism, the activity decay can be more or less pronounced. Being of particular concern is the pore-mouth poisoning at high Thiele modulus, conditions under which catalyst performance is drastically affected. The reagent concentration profiles allowed the explanation of the phenomena occurring at the particle scale, in particular the effectiveness factors, observed reaction rates, and poisoning factors' dependence on the Thiele modulus and fraction of the poisoned catalyst; it was found that such relationships are dependent on the mechanism of deactivation. (c) 2012 Canadian Society for Chemical Engineering