The unusual form of a phosphinous acid can be stabilized by strongly electron-withdrawing substituents such as trifluoromethyl and pentafluoroethyl groups. The less electron-withdrawing pentafluorophenyl group favors the phosphane oxide tautomer (Rf)2P(O)H in the solid state, whereas in solution a solvent-dependent equilibrium with the phosphinous acid tautomer (Rf)2POH is observed. The increasing donating ability of the solvent leads to an increasing amount of the corresponding phosphinous acid tautomer. In accord with quantum chemical calculations, the electron-withdrawing effects of the p-tetrafluoropyridyl and 2,4-bis(trifluoromethyl)phenyl groups exceed the pentafluorophenyl group and should therefore be ideally suited to stabilize the corresponding phosphinous acid tautomer (Rf)2POH. The syntheses of bis(tetrafluoropyridyl)- and bis[2,4-bis(trifluoromethyl)phenyl]phosphane oxide enabled the investigation of the solvent-dependent tautomerism by NMR spectroscopy. Introduction of the tetrafluoropyridyl group shifts the tautomeric equilibrium significantly towards the phosphinous acid. Surprisingly, the comparably electron-withdrawing but more bulky 2,4-bis(trifluoromethyl)phenyl group favors the oxide tautomer. The experimental results have been confirmed by DFT calculations. In summary, electron-withdrawing substituents stabilize the phosphinous acid tautomer, whereas it is destabilized by space-demanding groups by an increased C–P–C angle.