The effect of hydrogen and carbon monoxide prereduction on the initial activity and deactivation of chromia/alumina was investigated in isobutane dehydrogenation. Measurements were done at 580 degrees C in a fixed bed reactor and by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with mass spectrometry. Prereduction with hydrogen decreased the dehydrogenation activity compared to an isobutane-reduced catalyst, and prereduction by carbon monoxide increased the cracking activity. The catalysts deactivated with time on stream due to formation of carbon-containing deposits: carboxylates and aliphatic and unsaturated/aromatic hydrocarbon species. Prereduction affected the rate of coke deposition but not the nature of the species formed. The observed effects were attributed mainly to the hydroxyl groups formed during hydrogen prereduction and to the carbonate and formate species formed during carbon monoxide prereduction. Furthermore, the carbon monoxide-prereduced surface possibly contained a higher number of active chromium sites unselective for dehydrogenation.