The potential effects of water on the conversion of Lewis into Bronsted acid sites and on the competitive adsorption with hydrocarbons for acid sites during hydrocarbon cracking with zeolites have been investigated by means of in situ infrared spectroscopy (IR) at reaction temperatures, as well as by studying the kinetics of n-hexadecane cracking. Both in situ IR and kinetic results show that under the cracking reaction conditions neither an increase of Bronsted acidity nor a competing adsorption effect of water occurs. On the contrary, the role of water is the same as that of nitrogen, i.e., to act as diluent, improving feed hydrocarbon dispersion and increasing the relative rate of unimolecular cracking versus bimolecular cracking and hydrogen transfer reactions. We also show that the mechanistic information achieved by spectroscopic and kinetic studies can be directly extrapolated to explain the effects observed when cracking industrial vacuum gas oil feed using water as dispersant. (C) 2003 Elsevier Inc.