The relative emission indices of toxic substances (CO, NO,, HC, ...), one <Pi,i - in relation to the fuel consumption, and the other, - to the power output, are defined. A general application of these indices for engine evaluation on the basis of toxic substances emitted, is proposed. The defined relative indices and ip^i depend on the operating factors, and by that explain die basis of the problem of toxic substances emission. For illustration, selected emission indices (for carbon monoxide CO, nitrogen oxides NO,, and hydrocarbons HC) in the whole operation range of Diesel engine SW4(X) and spark ignition engine 126A1.076/E, are presented. Additionally, as an instance, polycyclic aromatic hydrocarbons (PAH) were determined by means of the gas chromatography. Qualitative determination of 33 substances and quantitative determination of 13 substances were carried out. The developed algori±m and obtained results can be used to quantitative prediction of exhaust gas emission for the driving cycles and determination of the most appropriate running conditions of the tested engines.