The activity of an alumina-supported vanadium oxide catalyst was studied in the dehydrogenation of i-butane and n-butane at 580 degrees C under atmospheric pressure. The dehydrogenation reaction of the butanes to butenes and hydrogen was found to be catalysed by the reduced state of vanadium oxide (V4+ Or V3+, Or possibly both). The activity of the catalyst was therefore studied in detail after reduction with H-2, CH4, and CO. The activity was highest after CO reduction, even though the average oxidation states after the three reductions were similar. It was concluded that the main reason for the lower activity after H-2 and CH4 reduction was the formation of OH groups that decreased the number of active sites for the dehydrogenation reaction. The primary cause of the catalyst deactivation during dehydrogenation was the formation of cake.