Short exposure of HZSM-5 zeolites to water vapor at high temperatures (mild steaming) enhances the rates of alkane cracking and dehydrogenation as well as the rates of exchange between H-2 and D-2. Solid state Al-27 MAS NMR results show that mild steaming leads to metastable aluminum oxide species, hypothesized to be partially framework-bound. Combining double quantum magic-angle spinning nuclear magnetic resonance experiments and kinetic analysis of cracking reactions allows us to conclude that the presence of these aluminum oxide clusters in the vicinity of Bronsted acid sites leads to an increase in the activation entropies during alkane cracking, induced by increasing steric constraints. Prolonged steaming results, in contrast, in the extraction of framework Al, which subsequently forms aggregated extra-framework Al oxide species partly blocking Bronsted acid sites and partly deposited at the outer surface of the crystallites. (C) 2018 Elsevier Inc. All rights reserved.