The presence of H2O introduced significant changes in the catalytic behavior of Co-ZSM5 during NOx reduction with propane at 500 degrees C. Analysis of fresh and spent catalysts using ultraviolet-visible spectroscopy and temperature-programmed reduction in H-2 were consistent with a deactivation mechanism involving nucleation growth of CoOx species. At short times on stream (TOS <= 12 h), the changes in activity (including an increase in activity) were caused by migration of Co species toward Co-free binding sites in the zeolite. At TOS of the order of tens of hours, CoOx, moieties nucleated and then grew to form small crystallites Of Co3O4 inside the zeolite channels. This process resulted in irreversible deactivation of the catalysts. The presence of a residual catalytic activity was ascribed to the existence of a hard-to-reduce Co2+ fraction. Structural characterization with X-ray diffraction, Si-29 and Al-27 magic angle spinning-nuclear magnetic resonance (MAS-NMR) showed that dealumination was not significant in our Co-ZSM5 samples. (c) 2004 Elsevier B.V. All rights reserved.