Cancels and replaces the first edition (1977). Establishes a method for the determination of basic constituents in petroleum products by potentiometric titration with perchloric acid in glacial acetic acid. Two variations of the method are described, using different titration solvent volumes and test sample masses. The ranges of base number values for which precision values for the method have been established are: unused oils - base numbers from 6 to 70; additive concentrates - base numbers from 5 to 300; used oils on which were employed the forward titrations - base numbers from 5 to 27.