The special attention in analysis of the sublayer extinguishing process was paid to results of full-scale tests. Low efficiency of the sublayer fire extinguishing system can be caused by a number of neglects which are allowed in processes of preparation and extinguishing. It is related to the choice of foaming agent, method of preparation of working solution, method of producing and transportation of foam, method of injection of foam into oil product. Some results of full-scale tests of the sublayer system during extinguishing of fires in tanks with oil or oil products, which were conducted on testing grounds of such cities as Perm, Novopolotsk, Astrakhan and Almetyevsk are presented in the article. It is also presented analysis of foreign information sources. On the basis of analysis of full-scale fire tests and comparison of obtained results, using model experiments, there were revealed the optimal parameters of the sublayer system in process of extinguishing of fires in oil tanks. Factors leading to decrease of fire extinguishing efficiency of the sublayer system are considered. The conclusion is that fire extinguishing efficiency of the sublayer system is decreased due to such factors as decrease of foaming agent concentration during preparation and storage of working solution, that affects spontaneous spreading of aqueous solution on the surface of oil product; an error in dosing of concentrated foaming agent in process of preparation of working solution; impossibility to obtain the foam with expansion rate not less than 3.5 using generators with consumption rate of more than 10 l/s and pressure (9+0.5) atm; application of foam generators with back pressure valve placed on air ejection line for creation the pressure necessary to open the valve and to rupture safety membrane; absence of a foam sampling procedure, while generator is working, which exclude capture of air by falling stream of foam; application of foam with expansion rate lower than 4.0; maintaining of optimal length of a foam line to obtain the foam with homogeneous structure; application of distributed scheme of foam nozzles location to prevent formation of local areas with raised layer of oil and to decrease the capture degree of foam by descending oil streams; providing the long period before burnback of oil product by heated tank walls due to loss of foam insulating properties.