In summary, we have examined the two-water catalyzed tautomerization of phosphinous acid (1) and model phosphinylidene compounds, each serving as representative cased of phosphinates (2), H-phosphonates (3), H-phosphinates (4) and secondary phosphine oxides (5). Depending on the systems, our rate constant calculations, including the multidimensional small-curvature tunneling approximation, show that tunneling significantly accelerates the gas-phase semiclassical reaction rates by ca. threefold to as large as two orders of magnitude at 298.15 K. The predicted unusually high KIE (kH/kD), and the concave Arrhenius graph of the tunneling rate constants around the room temperature regime, while not reaching a flat plateau as the temperature is lowered further, reveals the dominance of proton tunneling and its occurrence from the vibrational excited level by a thermally-activated tunneling process. We have also shown that for fluorinated secondary phosphine oxides, the rapid tunneling rate and short half-life time for the forward P(V)2w → P(III)2w reaction results in the fleeting existence of then P(V) tautomer, thus explaining the possible reason why only the P(III) form was previously detected by IR and NMR spectroscopy. Furthermore, we tested the influence of solvent (using a polarized continuum model) on the tunneling rates and predicted an influential role of tunneling for the systems with electron-donating groups 1-4. In contrast, due to low barriers, the semiclassical over-the barrier process becomes dominant for fluorinated systems 5 in a highly polar solvent. The present computational study thus elucidates the influential role of the tunneling effect in a family of model phosphinylidenes, thereby making it into the gallery of water-assisted tautomerization processes driven exclusively via a tunneling mechanism.