The interrelation between internal diffusion and deactivation is examined. The study is centered on the copper catalyst used in the water-gas-shift reaction (WGSR). This catalyst is deactivated by the presence of chlorine in the feed. A Langmuir-Hinshelwood model for the reaction kinetics is considered. With a high diffusional resistance, the poison is eliminated inside the pellet, and this explains the observed inhomogeneous deactivation of the industrial bed. The life of the catalyst increases when diffusional resistance for the poison increases. A general equation for the effectiveness factor, which includes the catalyst decay, is defined.