This method makes it possible to determine in petroleum products all the constituents which are easily saponified under the test conditions. Since sulfur compounds, phosphorus, halogens and certain other elements which are sometimes added to petroleum products also react with alkali or acids, the results obtained are influenced by these extraneous products. This possibility must be taken into account when interpreting the results obtained on products which contain these compounds, on used internal combustion engine crankcase oils, on used turbine oils and generally on all used oils. Note 1. — The products referred to above, which are not normally considered saponifiable matter, include inorganic acids and certain organic acids, most non-alkaline soaps, etc. The presence of such products gives a saponification index higher than that of the saponifiable materials for which the method was originally intended. The smell of hydrogen sulfide, which is released during the test towards the end of the back titration, is an indication that some type of active sulfur compound is present in the sample. Other active compounds of sulfur, chlorine, and phosphorus, as well as other compounds that may interfere, do not give this odor during the test. In this case the quantity of fatty acids actually present is probably best obtained by gravimetric determination.