Describes the procedure for the determination of the acidity of unused and used electrical mineral insulating oils. The method may be used to indicate relative changes that occur in a mineral insulating oil during use under oxidizing conditions regardless of the colour or other properties of the resulting mineral oil. The acidity can be used in the quality control of unused mineral oil. As a variety of oxidation products present in used mineral oil contribute to acidity and these products vary widely in their corrosion properties, the test cannot be used to predict corrosiveness of a mineral oil under service conditions. The acidity results obtained by this test method may or may not be numerically the same as those obtained by colorimetric methods, but they are generally of the same magnitude.