Nonenzymatic peroxidation occurs through the action of reactive oxygen species (ROS), specifically hydroxyl (HO) and hydroperoxyl (HO) radicals, which initiate the oxidation of polyunsaturated fatty acids. Other initiators of lipid peroxidation include ozone (O), nitrogen oxide (NO), nitrogen dioxide (NO), and sulfur dioxide. The process of nonenzymatic peroxidation can be divided into three phases: initiation, propagation, and termination. 
During the initiation phase, fatty acid radicals are generated, which can propagate peroxidation to other molecules. This occurs when a free radical removes a hydrogen atom from a fatty acid, resulting in a lipid radical (L) with an unpaired electron. 
In the propagation phase, the lipid radical reacts with oxygen (O) or a transition metal, forming a peroxyl radical (LOO). This peroxyl radical continues the chain reaction by reacting with a new unsaturated fatty acid, producing a new lipid radical (L) and lipid hydroperoxide (LOOH). These primary products can further decompose into secondary products. 
The termination phase involves the interaction of a radical with an antioxidant molecule, such as α-tocopherol (vitamin E), which inhibits the propagation of chain reactions, thus terminating peroxidation. Another method of termination is the reaction between a lipid radical and a lipid peroxide, or the combination of two lipid peroxide molecules, resulting in stable nonreactive molecules. Reinforced lipids that become part of the membrane if consumed with heavy isotope diet also inhibit peroxidation.