Sulfated zirconia with 10 wt.% nominal sulfate loading was synthesised, characterised and tested for n-butane isomerisation activity varying butane diluents such as CO2, CO and nitrogen, and reaction temperatures. Results revealed that a fresh, air-pre-treated catalyst was poisoned by CO2, and exhibited low catalytic activity, whereas an air-regenerated catalyst showed high butane conversion with negligible catalyst deactivation in presence of CO2. Incorporation of CO in the feed stream poisoned the catalyst immediately and removal of CO from the feed regenerated the catalyst activity. The reversible poisoning effect of CO was prominent at low temperature and was absent at elevated temperatures. It seems that the surface oxy species of sulfated zirconia, both the lattice and the adsorbed, play an important role in butane isomerisation and catalyst poisoning by CO2, whereas, formation of surface stabilised adspecies causes the negative effect of CO. (C) 2002 Elsevier Science B.V All rights reserved.