The tautomeric behaviour of five secondary phosphane oxides (SPOs) with different electronic properties has been investigated by NMR and IR spectroscopy, density functional theory calculations and X-ray structural analysis. Proof is given that only with strong electron-withdrawing groups on the phosphorus atom can the relevant trivalent phosphinous acid be observed in the equilibrium, whereas in the case of electron-rich phosphane oxides the pentavalent species dominates. For a detailed analysis of the IR spectra, the de-convolution programme BTEM has been successfully applied.