The effect of non-uniform catalytic activity distribution on product selectivity in the consecutive reaction scheme A → B → C has been investigated for a single catalyst pellet and also for a fixed bed reactor containing non-uniform catalyst pellets, taking into consideration independent variations in the catalytic activities for the reaction steps A → B and B → C and the non-zero concentration of the product B at the catalyst surface. Except with very small values of Thiele moduli, on-uniform activity distribution has a significant effect on selectivity. For a single catalyst pellet a combination of shell type catalytic activity distribution for the reaction A → B and core type distribution for B → C appears to give the best product selectivity. In a packed bed reactor the local selectivity decreases along the reactor, depending on the modified Damkohler number. © 1991.