The mechanism of antioxidant action in vitro is discussed, focusing on the autoxidation of lipids, a free radical chain reaction. The reaction between lipids and oxygen, known as autoxidation, involves initiation, propagation, and termination steps. Initiation can occur through direct reactions between a lipid and oxygen, but this is unlikely due to spin angular momentum conservation. More probable initiation processes include reactions between a metal catalyst and a lipid, or the decomposition of hydroperoxides. Hydroperoxides are formed during propagation reactions and can also be generated by reactions with oxygen in its singlet excited state or through enzyme-catalyzed reactions. Triplet oxygen can be converted to singlet oxygen in the presence of photosensitizers, which absorb light and transfer energy to oxygen, leading to the formation of singlet oxygen. Singlet oxygen can then react with lipids to form hydroperoxides. The autoxidation process is inhibited by antioxidants, which can terminate the chain reaction by combining with radicals. The energy required for the reaction between a lipid and oxygen is high, ranging from 146 to 272 kJ/mol. The initiation of autoxidation is a critical step in the degradation of lipids and is influenced by various factors, including the presence of metal catalysts, hydroperoxides, and photosensitizers. Understanding the mechanism of autoxidation is essential for developing strategies to prevent lipid degradation in food and industrial products.The mechanism of antioxidant action in vitro is discussed, focusing on the autoxidation of lipids, a free radical chain reaction. The reaction between lipids and oxygen, known as autoxidation, involves initiation, propagation, and termination steps. Initiation can occur through direct reactions between a lipid and oxygen, but this is unlikely due to spin angular momentum conservation. More probable initiation processes include reactions between a metal catalyst and a lipid, or the decomposition of hydroperoxides. Hydroperoxides are formed during propagation reactions and can also be generated by reactions with oxygen in its singlet excited state or through enzyme-catalyzed reactions. Triplet oxygen can be converted to singlet oxygen in the presence of photosensitizers, which absorb light and transfer energy to oxygen, leading to the formation of singlet oxygen. Singlet oxygen can then react with lipids to form hydroperoxides. The autoxidation process is inhibited by antioxidants, which can terminate the chain reaction by combining with radicals. The energy required for the reaction between a lipid and oxygen is high, ranging from 146 to 272 kJ/mol. The initiation of autoxidation is a critical step in the degradation of lipids and is influenced by various factors, including the presence of metal catalysts, hydroperoxides, and photosensitizers. Understanding the mechanism of autoxidation is essential for developing strategies to prevent lipid degradation in food and industrial products.