Compressed natural gas (CNG), its mixt. with H-2 (90% by vol. of CNG and 10% of H-2) as well as a gasoline were used sep. as fuel for various engine loads (50-100%) and various rotations of the engine crankshaft (1000-4000 rpm). Concns. of CO, NOx and hydrocarbons in the flue gas for each type of fuel used were detd. according to existing procedures using an IR, flame ionization and chemiluminascence detector, resp. The CO emission from combustion of gaseous fuels was very low and independent on engine load and crankshaft rotation. The H addn. to CNG resulted in decreasing the concns. of CO, hydrocarbons and NOx in the exhaust gas when compared to gases from CNG combustion. The gasoline combustion resulted in the higher emission than the combustion of gaseous fuels.