Abstract The aim of the thesis is to expand the knowledge of the catalytic properties of platinum-loaded zeolite catalysts in the reduction of NOx by hydrocarbons. The work is divided into three parts. First the recent literature of zeolite catalysts has been introduced, secondly the adsorption capacity, activity, and acidity of the catalysts have been studied by TPD and IR techniques, and thirdly the derived reaction mechanisms based on the obtained data are presented. Parent and 1 wt-% Pt-loaded ZSM-5, Beta, Y, and Ferrierite zeolite catalysts have been studied in the C3H6-assisted reduction of NO. The Pd/Al2O3-based catalyst was used as a reference material for the reaction mechanistic studies. Several experimental techniques (in situ DRIFT, activity measurements, CO chemisorption, N2 physisorption, TPD, and TEM) have been used for the characterisation of the catalysts properties. The IR technique was used as the main technique for the determination of activities, surface species, and the acidic properties of the zeolite-based catalysts. The activity studies carried out by the gaseous FTIR technique provide information on the desired reaction products as well as the undesired by-products. The detection and identification of the surface species as well as the reaction intermediates formed were done by the DRIFT method. The activity experiments indicate the effectiveness of the Pt-loaded zeolite catalysts. The reduction of NO was found to decrease in the order: Pt/Beta > Pt/Y > Pt/Ferrierite > Pt/ZSM-5 in the conditions with excess O2. Platinum can be concluded to have an effect on O2 and NO dissociation. Oxidation reactions of NO to NO2 and propene to CO2 were observed to be more intense over the platinum-loaded zeolites than over the parent zeolites. In this work the reaction mechanisms for the C3H6-SCR of NO were derived over the Pt-loaded zeolite as well as the Pd/alumina catalysts based on the data obtained by DRIFT and activity experiments. The kinetics for the NO reduction by CO over Pd/Al2O3 was also derived. With the methods employed, the mechanistic steps over the Pt-loaded zeolites and Pd/Al2O3-based catalysts could be derived quite precisely and easily for C3H6-SCR of NO. Reaction routes were determined to go via different formations of intermediates over the two catalysts, i.e. via organonitrogen and isocyanate routes, respectively. The IR techniques were discovered to be effective tools in applied engineering studies.