Abstract The single-pellet diffusion reactor has proven to be a particularly useful tool to diagnose the mechanism of catalyst poisoning. Previous work was done with main reactions of first, second and 1/2 order and behavior was found to be similar for reactions of different orders. However, a zero order main reaction in this reactor presents a singular behavior not observed previously. These singularities are illustrated using the limiting poisoning models referred to as pore-mouth, uniform and core poisoning: cases which can be solved analytically. From these analyses, it is clear that the single-pellet diffusion reactor can still be used to discriminate among the various poisoning mechanisms causing deactivation. However, the range of Thiele parameter over which the measured centerplane concentration gives useful additional information is reduced to values of the effective Thiele parameter less than √2.