The effect of oxygen-containing additives on the toxic emission of a SI engine, has been studied. The relationship between the chemical structure of the additives, their oxygen content and molecular mass, and the chemical composition of the exhaust gas toxic compounds (CO, HC and NOx), has been established. More than 20 samples of fuel blends containing oxygenates, were tested. The tested oxygenates represent three large chemical structure categories which can be found in the chemical industry wastes-alcohols, ethers and ketones. The tests were performed using a four-cylinder, spark-ignition engine. In order to determine the optimum types of fuel additives for the suppression of carbon monoxide, hydrocarbons and nitro-oxygens, fuel blends containing 10% oxygenates with different oxygen content (from 9% to 50%) and molecular mass (from 32 to 170) were examined. It was found that with 30 to 50% additive oxygen content, the CO and HC can easily be reduced by some 60% and 30% respectively, while this was always associated with an increasing in the NOx emission by some 5-10%. The effect of the oxygenate additive on the exhaust gas emissions, was found to be strongly dependent not only on the additive oxygen content, but also on the chemical structure of the additive. The CO reduction activity is in the order ketones>alcohols>ethers, while the HC reduction activity is in the order ketones>ethers>alcohols. The NO x increasing reactivity is in the order ethers>ketones> alcohols.