Specifies a test for the oxidation stability of unused inhibited mineral insulating oils by measurement of the induction period. The method is intended for testing the continuity of oil supplies. The results do not necessarily provide information of the performance in service. - The oil sample is maintained at 120 degrees centigrade in the presence of a solid copper catalyst coil whilst bubbling through a constant flow of oxygen. Volatile oxidation products are absorbed in aqueous alkali solution containing phenolphthalein. The time required to develop a volatile acidity corresponding to a neutralization value of 0.28 mg KOH/g oil is measured.