Oxidative phosphorylation is the process of ATP synthesis that occurs when electrons pass through the electron transport system (ETS). In this process, electrons from NADH and FADH₂ are passed from one carrier to another via complexes I to IV in the electron transport chain present in the inner mitochondrial membrane.

These electrons are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate. The energy utilized for this phosphorylation comes from oxidation-reduction reactions, unlike photophosphorylation where light energy is used. Oxygen acts as the final hydrogen acceptor, driving the whole process.