Operation of compression-ignition engines (6C107), used as propulsion of mining locomotives, was analysed in respect of toxic substances emission. To this end, relative emission indices, defined in relation to fuel consumption and to power output, were taken into consideration. These indices are appropriate quantities because they take also engine operation parameters into account. The experiments were aimed at verifying if the engines meet very strict mining emission standards. The emission was investigated before and after the catalyst. Thus, changes of the indices within the catalyst and conversion rate of the toxic substances were evaluated. The analysis and obtained results were used to:determination of the most appropriate running conditions of the tested engines,deduction of the methods of gaseous emission reduction at the most unfavourable operation ranges of the engines.Proposed conception and presented method of considerable gaseous substances emission reduction in the whole operation range of the Diesel engine consists in using oxidation catalyst and cutting-off fuel supply to selected cylinders of the engine.This paper was presented at the ECOS’99 Conference in Tokyo, June 8-10, 1999