A study on reducing NO in exhaust fumes of diesel engines has been carried out; simulated exhaust gas of engines fuelled with high sulphur content heavy oil was used for the experiments reported in this paper. The feasibility of thermal reduction of NO by addition of ammonia in presence of SO2 has been investigated. High reduction levels at NO and O-2 concentration values typical of exhaust gas have been obtained, more than sufficient to reduce NO outlet concentration values so as to fall within the Italian law limits on emissions. The efficiency of the process turned out to be dependent on temperature, ammonia/NO inlet concentration ratio, residence time in the high-temperature zone, NO and O-2 concentrations. the aim of the study was to get information on optimal reaction conditions and process yield, as well as on SO2 influence on the process. The reaction products were also characterized, in order to investigate on the destiny of excess ammonia sent to the reactor: N-2 turned out to be the main product. Moreover, for a reactor inlet molar concentration ratio [NH3](in)/[NO](in) less than or equal to 1.5 and an oxygen content in the gas higher than 4% the concentration of unreacted ammonia in the gases leaving the reactor was lower than 50 vpm. Only small concentrations of N2O were detected.