A recent numerical studies by Yakhnin and Menzinger (Proc. 8th Int. Symp. on Catalyst Deactivation, Brugge, Belgium, 10-13 October, Vol. 126, 1999, pp. 291-298) suggests that activity inhomogeneity in a catalyst bed can create larger temperature excursions after changes in feed temperature than would be encountered in beds with homogeneous catalyst activity. Until now, this finding has not been confirmed experimentally. Inhomogeneous catalyst activity was simulated by constructing a packed bed reactor by successive layers of catalyst and the inert support. Oxidation of carbon monoxide (CO) with oxygen in a nitrogen carrier gas over an industrial 0.2 wt% Pt/gamma -alumina catalyst was employed. This reaction is exothermic and inhibited by CO. Step changes and periodically varying inlet temperature were used to generate disturbances. Both wrong-way behaviour and differential flow instability were observed. Temperature excursions in the layered bed significantly exceeded those in a bed of homogeneous activity at the same operating conditions.