Oils that are rich in unsaturated fatty acids frequently undergo oxidation at ordinary temperature. The oxidation proceeds slowly and forms short chain fatty acids (C₄ to C₁₀) and aldehydes, which give a rancid taste and odour to the fats. This process is also known as oxidative rancidity because it undergoes autoxidation.

Autoxidation takes place by free radicals in which the α-methylene group is first attacked. A hydrogen atom is removed and this leads to a chain of reactions leading to oxidation.

Oxidative rancidity takes place more frequently in animal fats than in vegetable fats. This is because natural antioxidants such as tocopherols, phenols, and napthols present in the vegetable oils will check autoxidation.