The purpose of the present study is the investigation of the composition of the ash of fuels T-2, TS-1, T-1, and DA. Hydrocarbon fuels contain compounds with ash (metals and metalloids) elements--organo-metallic compounds--in very small quantities. These elements, in the form of oxides, are concentrated in the ash after combustion of the fuels. In spite of the low content of compounds with ash elements in fuels, their effect on the operational properties of the fuels is extraordinarily great. Intensified gas corrosion of jet engines and gas turbine units is observed in the presence of compounds with ash elements. An increase in the content of organo-metallic compounds and other compounds with ash elements in motor-engine fuels entails an increase in the amount of carbon deposits in the combustion chamber of piston engines. In addition, it has been established that compounds with ash elements promote the formation of insoluble sludge and resins during heating of the fuels.