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In this work, the deactivation of a CrOx/Al 2 O 3 dehydrogenation catalyst in an industrial reactor has been systematically studied under cyclic dehydrogenation−regeneration conditions from start up to end of run. The results showed that the CrOx/Al 2 O 3 catalyst deactivation occurred through three major processes. The first process is related to the complex transformation of surface chromium species, the second process refers to the phase transformation and/or sintering of support alumina, and the third process is the interfacial CrOx/Al 2 O 3 transformation into the solid solution of α-(Al, Cr) 2 O 3 . In addition, the reduction of the chromium oxide surface area and the migration of Cr 3+ into the alumina support resulted in the increase of the surface Cr 6+ /Cr 3+ ratio at the end of the run which increased the specific catalyst acidity and reduced the catalyst selectivity. The main causes of irreversible deactivation of the CrOx/Al 2 O 3 catalyst are the decrease of chromium oxide surface area, the formation of α-(Al, Cr) 2 O 3 in the support, and the increase of surface Cr 6+ /Cr 3+ ratio.