Irreversible deactivation of chromia/alumina dehydrogenation catalysts was investigated by deactivating the catalysts (4 wt% Cr) through prolonged calcination in air at temperatures up to 1200degreesC. Nitrogen physisorption, X-ray diffraction, diffuse reflectance spectroscopy in the UV-visible region, X-ray photoelectron spectroscopy, electron spin resonance spectroscopy, and catalytic measurements revealed that two deactivation mechanisms were responsible for the formation of catalytically inactive alumina-incorporated Cr3+ species: (i) entrapment of Cr3+ ions inside the alumina support during sintering of the alumina support and (ii) migration of Cr3+ ions into the alumina support. Chromia that had been impregnated on alumina inhibited the phase transformation from gamma- to alpha-alumina. The presence of gamma-alumina, in turn, hindered the migration of Cr3+ ions into alumina. The turnover frequency was higher for multinuclear Cr3+ than for isolated Cr3+ sites. The phase of the alumina support did not appear to affect the activity of the surface Cr3+ species.