The structural collapse and the reconstruction of Bronsted acid sites in zeolite Y were investigated by combining propane monomolecular cracking with various structural characterization techniques. Destruction of Bronsted acid sites, together with a loss of catalytic activity, was found after calcination and exposure to moisture at room temperature. A loss in long-range structure was observed by XRD. Adsorption of NH3 before exposure to air prevents the structural collapse of Bronsted acid sites and the loss of catalytic activity. The collapsed Bronsted acid sites and long-range order recover fully after thermal treatment with NH3 at moderate temperature, as does the catalytic activity. Reheating of HY causes further structural collapse. Water molecules are involved in structural destruction, and also play a role in structural recovery with ammonia. The structure of zeolites is very sensitive to temperature and presence of moisture. Because various characterization methods are preceded by various pretreatment methods, the zeolite structure probed by these methods is not necessarily the same. (c) 2006 Elsevier Inc. All rights reserved.