Irradiation of single crystals of phenylphosphinic acid, C₆H₅P(H)(O)OH, gives rise to a radical to which the structure C₆H₅P(O)OH is ascribed. The radical exists in four different symmetry-related orientations and the average principal values of the g-tensor and of the ³¹P coupling tensor are 2·0023, 2·0034, 2·0034 and 1865, 1346, 1320 MHz.

Irradiation of the polycrystalline acid gives identical results for the E.S.R. parameters. Irradiation of various polycrystalline salts of phenylphosphinic acid gives rise to spectra attributed to C₆H₅PO₂^-, the phosphorus analogue of the nitrobenzene radical anion. The E.S.R. parameters are similar to those of the radical arising from the corresponding acid but vary with the nature of the cation. They indicate that unlike C₆H₅NO₂^- the radical is non-planar and has the unpaired electron largely localized on the phosphorus atom.