The study aimed to determine the extinction mechanism for composite fuels' flames by foam feeding into the fuel layer. Octane and heptane containing preset concentration of isopropyl alcohol were used as a composite fuel. The main research method was the analysis of experimentally obtained dependence of flame extinguishing on the intensity of foam supply. The results showed, that the most significant factor of extinction is the contact interaction between foam and composite fuel. The performed researchers revealed the principle ability of under-layer method for extinguishing of fires in reservoirs with composite fuels, where the concentration of alcohol doesn't exceed 10% by mass.