AT present, the orthodox view of the coupling of phosphorylation to electron and hydrogen transfer in oxidative and photosynthetic phosphorylation stems from knowledge of substratelevel phosphorylation1• 2• It is based, consequently, on the idea that water is expelled spontaneously between two chemical groups, A and B, by the formation of a strong bond (of low hydrolysis energy), and that subsequent or simultaneous oxidation or reduction of AB to AB* can result in a weakening of the bond, popularly written A"" B*, so that adenosine triphosphate (ATP) can be synthesized by coupling the opening of A"" B* tu tho closing of the'high-energy bond'between adenosine diphosphate (ADP) and phosphorus through group transfer systems of appropriate substrate and oxido-reductioncarrier specificities•-•. There are a number of facts about the systems catalysing oxidative and photosynthetic phosphorylation that are generally acknowledged to be difficult to reconcile with this orthodox (chemical) view of the mechanism of coupling. For example:(a) The hypothetical'high-energy'intermediates (for example, reduced diphosphopyridine nucleotide (DPNH)""?, reduced flavin adenine