In this article, we systematically investigate the two water-catalyzed tautomerizations of phosphinous acid (1) and phosphinylidenes substituted at the R1 and R2 positions, each representing a class of phosphinates (2), H-phosphonates (3), H-phosphinates (4) and secondary phosphine oxides (5) (see Scheme 1B). Using canonical variational transition state theory inclusive of multidimensional small-curvature tunneling, we shall demonstrate the significant tunneling contribution to the two-water bridged tautomerization process of 1-5 in the gas phase as well as in solvent media. The concave Arrhenius plots and abnormally high KIE reveal unambiguous fingerprints of tunneling at room temperature. We also explore the kinetics of reversibility of the experimentally reported fluorinated systems 5 and propose a possible reason for the elusive nature of