The H 3 P(O) → H 2 P(OH) tautomerism is addressed by experimental and DFT approaches. The process, disfavored for the free molecule, is easier over metal fragments of the type {CpRu II L 2 } n (L = uncharged or anionic phosphine ligand), with an energy barrier reduced to one fourth. The free H 3 P(O) molecule is a very weak acid and hardly a proton migrates intra-molecularly towards the oxo atom, as expected for classic acid-base reactions. Rather, some electron density of the highly covalent P–H bond remains anchored to the H atom at least up to the TS with the barrier originated from the electronic repulsion with the approached O lone pair. Beyond TS, the H atom transforms into a proton after having released its electron portion at the P atom (lone pair). The calculations show the experimentally undetected intermediate [CpRu(PR 3 ) 2 (H)(H 2 PO)] n , at which the metal has induced a P–H oxidative addition. Consistent behaviors are found for all the molecules H n (OH) 3−n P(O) (n = 3, 2, 1), whereas some anomalies have been experimentally observed with the anionic TPPMS coligands [TPPMS = PPh 2 (m-C 6 H 4 SO 3 ) − ], used to favor the chemistry in water. In particular: i) the reaction with H 3 P(O) indicates that the product [CpRu(TPPMS) 2 {H 2 P(OH)}] − exists in two isomeric forms; ii) the tautomerization of H(OH) 2 P(O) is uniquely inhibited. Ad-hoc DFT calculations indicate that the features are attributable to the strong H-bonding networks between the sulphonate substituent and OH group(s) and water as well.