Synthetic and natural antioxidants are used in the biodiesel and food industries to increase product shelf lives. Rancimat, which is regulated by EN 14112, is the official analytical method used to determine the oxidation stability of biodiesel, oils and fats. It involves subjecting samples to a temperature of 110 °C for prolonged periods. Some antioxidants decompose or volatilize below 110 °C, leading to false results. A TG/DTA technique with both dynamic and isothermal (110 °C) analysis methods was used to evaluate the thermal stability of commercial antioxidants. Synthetic antioxidants exhibited thermal resistances in the following order: PG > TBHQ > BHA > BHT. Initial thermal decomposition temperatures were lower than 110 °C, and BHT, BHA and TBHQ volatilized during the first few hours of the analysis. Natural antioxidants are resistant to heat and displayed stabilities in the following order: α-tocopherol > caffeic acid > ferulic acid > gallic acid. Subjecting chelating agents to a temperature of 110 °C for prolonged periods resulted in the following order of thermal stabilities: ascorbic acid > citric acid > EDTA. The initial thermal decomposition temperatures for both gallic acid and EDTA were lower than 110 °C.