The wedge-layering theory of foulant deposition has been extended to analyse the impact of deactivation by fouling upon the simple consecutive reaction A + B + C. The changes in diffusional resistances and surface area as the accumulating foulant deposit interacts geometrically with the pore volume contairzd in distributed catalyst pore sizes produces remarkably complex patterns of selectivity behaviour as deactivation proceeds in a plug flow reactor. The yield of intermediate is shown to theoretically follow a rising-falling-rising pattern when the deactivation is pore mouth plugging dominated. This behaviour is due to mouth plugging initially sealing-off low yield small diameter pores, with a subsequently falling yield as the constricted mouths of larger pores introduce additional diffusional resistances, and a final improvement in yield as deactivation becomes so complete that the conversion achieved approaches zero. Complex chemical effects are thus seen to arise from simple physical processes within pores. Experimental results for the yield of benzene intermediate in the cracking of cumene over a commercial FCC catalyst match these theoretical observations.