Experimental research on the diesel engine 6C107 equipped with selected oxidation catalytic converters was carried out. Specific emissions of toxic substances were investigated in the whole operation range of the engine before and after catalysts. Thus, changes of the emission indices within the catalysts and conversion efficiencies of the harmful substances were evaluated. Besides, temperature threshold of the catalytic action was determined too. Apart from chemical efficiency of the converters, their resistance to exhaust gas flow also is an essential problem. Therefore selected quantities of exhaust gas flow through the catalysts were determined and analysed together with their termochemical efficiency.