Abstract: The invention relates to an agent for increasing the octane number of a gasoline automotive fuel, which agent is a combination of alcohol and a product of reaction between a carbonyl compound and a compound containing at least two hydroxyl groups enabling to form cycles with carbonyl compounds, or is the mixtures of the above mentioned products. In the preferred variant, mono- or oligosaccharides or diatomic, triatomic or polyatomic alcohols are used as the compounds containing at least two hydroxyl groups enabling to form cycles with carbonyl compounds. Pentoses, preferably xylose or arabinose or hexose, substantially glucose, and the mixtures thereof are used as the monosaccharides. Glycols, for example, ethylene glycol are used as diatomic alcohols, glycerin is used as triatomic alcohols and erythritols, for example, pentaerythritol are used as polyatomic alcohols.