The paper presents a comparison of the catalytic properties of two types of catalytic converters (containing platinum and oxide active coatings) in the aspect of a reduction of nitric oxides with ammonia. The tests were performed for three concentrations of ammonia in the exhaust gases of a diesel engine. The work contains images of the topography of the active surface of the catalytic converters and the results of its physical and chemical analysis. The changes of the conversion rates of the nitric oxides have also been presented in the converter temperature range from 150 to 550 °C along with the results of calculations of the shares of NO2 in NOx for all analyzed concentrations of ammonia in the exhaust gases.