Gives a method for the determination of acidic constituents in products soluble or nearly soluble in mixtures of toluene and propan-2-ol. It is applicable for the determination of acids whose dissociation constants in water are greater than 10_-9. The method may be used to indicate relative changes that occur in an oil during use under oxidizing conditions regardless of the colour or other properties of the resulting oil. No general relationship between bearing corrosion and acid or base numbers is known.