This is the most accepted of the three hypotheses and proposes that electron transport is coupled to ATP synthesis by a sequence of consecutive reactions, in which a high energy covalent intermediate is formed by the electron transport and subsequently is cleaved and donates its energy to form ATP. The hypothesis explains the direct chemical coupling at all stages of the process. In a redox reaction between A reduced and B oxidised, and a factor I is incorporated into the formation of an energy-rich structure A oxidised ~ I, where ~ indicates a linkage having an energy rich nature.

A red. + I + B oxi. → A oxi. –I + B red.

In subsequent reactions, an enzyme (E) replaces complex A oxidised~I to form an energy-rich E~I. Later, inorganic phosphate reacts with E~I to form phosphoenzyme complex E~P, containing the energy-rich enzyme–phosphate bond.

A oxi~I + E  A oxi + E~I
E~I+Pi  E~P + I

The enzyme phosphate component finally reacts with ADP to form ATP.

E~P + ADP  E + ATP