The initial activity of alumina-supported vanadium oxide catalysts with and without Mg and Zr was studied in the dehydrogenation of i-butane and n-butane at 580 degreesC under atmospheric pressure. The catalyst activity was evaluated, after reduction with H-2 and CO, as a function of the amount of the promotor. The dehydrogenation activity was highest after CO reduction, and the selectivity toward i-butene increased and the formation of coke decreased with the addition of Mg. According to X-ray photoelectron spectroscopy, electron spin resonance, temperature programmed reduction, and Fourier transform infrared measurements, during CO reduction, V3+ species and two kinds of V4+ species with different coordinations (A and B) were formed on the catalyst surface. During H-2 reduction V3+ and some V4+ (B) species were formed. Apparently both V4+ and V3+ species were active in the dehydrogenation, and tentatively, we conclude that the activity of V4+ (A) was higher than that of V3+. For the modified and unmodified catalysts, the oxidation states after reduction, both with H-2 and with CO, were similar and were not the main reason for the differences in activities. It is likely that the differences in activities induced by the promotors were due to the acidities of the catalysts, which had an influence on the side reactions of cracking, skeletal isomerization, and coke formation. The oxidative dehydrogenation during the first minutes on butane stream with calcined catalysts was also affected by the acidity of the catalyst.