Selective catalytic reduction of NOx on Ga3+ exchanged ferrierite zeolites (FER) using ammonia as reductant shows high rates of NO, conversion, although neither the catalytic reaction mechanism nor the role of the gallium is understood. We investigate the features of the structure of the catalyst and the mechanism of the catalytic reduction of NOx to N-2 using nuclear magnetic resonance (NMR) spectroscopy, complemented by temperature programmed desorption (TPD) and catalytic conversion measurements. The NMR studies are enabled by the diamagnetic nature of the gallium-exchanged zeolites in contrast to many of the transition metal exchanged zeolites which are often paramagnetic. Two different ferrierite samples were chosen for comparison, one containing significant quantities of K+ and Na+, the other being largely alkali-free. The gallium-exchanged zeolites and their parent materials were characterized using Si-29, Al-27, Ga-71, and Xe-129 NMR. Reactions run under batch-mode conditions using ammonia as the reducing agent clearly show the formation of nitrogen in the N-15 NMR spectra. Our results show that the acidity of the zeolite is more important for catalytic reduction than the presence of gallium when NO., is supplied to the catalyst as a 1:1 mixture of NO and NO2. (c) 2004 Elsevier B.V. All rights reserved.