The ratio of phosphate incorporated into ATP to oxygen atoms reduced to water; a measure of the efficiency of coupling of phosphorylation to oxidation. Based on the now-obsolete chemical coupling theory of oxidative phosphorylation, the passage of electrons down the electron transport pathway results at several points in the chemical synthesis of ATP. The number of phosphates that are fixed into ATP for every two electrons that pass from a substrate to reduce each oxygen atom of molecular O₂ (the P:O ratio) corresponds to the number of such points and should be an integer value. Experimentally, it was found to be between 2 and 3 when NADH was the donor of electrons and between 1 and 2 when succinate was the donor, and the theoretical integer values were set to 3 and 2 respectively. The chemiosmotic theory similarly predicts H⁺:O and H⁺:ATP ratios. Experimentally these appear to be 10 and 4 respectively when NADH is the substrate, equivalent to a P:O ratio of 2.5, and 6 and 4 respectively for FAD-linked substrates (e.g. succinate), equivalent to a P:O ratio of 1.5.