The chapter introduces the mechanism of antioxidant action in vitro, focusing on the spontaneous reaction of atmospheric oxygen with organic compounds, which leads to degradative changes that reduce the lifespan of many products, particularly polymers and lipids in foods. The importance of oxygen in the deterioration of rubber was recognized over a century ago, leading to extensive research on oxidative deterioration and its inhibition. The chapter discusses the autoxidation process, which is a free radical chain reaction involving initiation, propagation, and termination steps. The initiation step involves the formation of free radicals, typically through the decomposition of hydroperoxides or the reaction of a lipid molecule with a metal catalyst. The propagation step involves the conversion of free radicals into other radicals, while the termination step involves the combination of two radicals to form stable products. The energy of activation for the direct reaction of a lipid molecule with oxygen is very high, making it highly improbable. Instead, more probable processes include the formation of free radicals through the decomposition of hydroperoxides or the conversion of triplet oxygen to singlet oxygen via photosensitizers. The singlet oxygen then reacts with a lipid molecule to form a hydroperoxide, initiating the autoxidation process.The chapter introduces the mechanism of antioxidant action in vitro, focusing on the spontaneous reaction of atmospheric oxygen with organic compounds, which leads to degradative changes that reduce the lifespan of many products, particularly polymers and lipids in foods. The importance of oxygen in the deterioration of rubber was recognized over a century ago, leading to extensive research on oxidative deterioration and its inhibition. The chapter discusses the autoxidation process, which is a free radical chain reaction involving initiation, propagation, and termination steps. The initiation step involves the formation of free radicals, typically through the decomposition of hydroperoxides or the reaction of a lipid molecule with a metal catalyst. The propagation step involves the conversion of free radicals into other radicals, while the termination step involves the combination of two radicals to form stable products. The energy of activation for the direct reaction of a lipid molecule with oxygen is very high, making it highly improbable. Instead, more probable processes include the formation of free radicals through the decomposition of hydroperoxides or the conversion of triplet oxygen to singlet oxygen via photosensitizers. The singlet oxygen then reacts with a lipid molecule to form a hydroperoxide, initiating the autoxidation process.