The characteristics and mechanism of deposit formation in spray injection system of gasoline engines are discussed. A motorless unit and a method of evaluation of deposit forming tendency of automotive gasolines in spray injection systems are developed. The criterion of numerical evaluation of pollutants based on degree of darkening of spray injector bottom is proposed. The results of tests, by the proposed method, of individual hydrocarbons and their mixtures as well as of several gasolines of different producers are adduced. Among the individual hydrocarbons, unsaturated hydrocarbons and nitrogen-bearing compounds contribute the most to deposit formation, and additivity and synergy effect are not typical for the deposit formation process. Among the commercial gasolines, those containing MMA (monomethyl aniline) or a substantial quantity of catalytically cracked fractions and not containing detergent additives exhibit maximum deposit forming tendency. Minimal deposit formation is typical for gasolines containing detergent additives.