The phenolic antioxidants 2,6-ditertiary-butyl paracresol (DBPC) (also known as butylated hydroxytoluene (BHT)) and 2,6-ditertiary-butyl phenol (DBP) are the two most common oxidation inhibitors added to the electrical insulating (transformer) oil and mineral oil based lubricants. The typical recommended value of DBPC and DBP in fresh electrical insulating oil is approximately 0.3% by weight. These inhibitors prevent electrical insulating oil from oxidative degradation, and prolong the life of the oil. It is essential to maintain the optimum concentration level of inhibitors to ensure the proper functioning of mineral oil used in transformer units as an insulating or cooling agent. The depletion rate of the inhibitors in the oil is dependent on various factors such as the amount of oxygen available, soluble contaminants, catalytic agents, and temperature. Therefore, regular testing of the inhibitors in electrical insulating oil is necessary to ensure its reliable operation in high value assets such as transformer units. ASTM 2668 and IEC 60666 are the standard test methods, using infrared spectroscopy (IR) technology to monitor the concentrations of DBPC and DBP in electrical insulating oil. These test methods are used to determine if the new electrical insulating oil meets the specification for oxidation inhibitor initial concentration levels. They also monitor the concentration of inhibitors in the used oil. If the inhibitors have been depleted to a critical level, additional inhibitor can be added. Therefore, the standard test methods are essential for manufacturing control, specification acceptance, and to periodically monitor the level of inhibitors in used oils.