The E.S.R. spectra of phosphorus-centred radicals formed in the X-irradiation of single crystals of methylene diphosphonic acid are reported. The radicals are identified as phosphoranyl radicals, (HO)₂(O)P-CH₂-P(OH)₃ and the tricoordinated species OP(OH)₂, (HO)₂(O)P-CH₂-P(O)(OH). In the last radical coupling from the second ³¹P nucleus is also observed. Attempts are made to correlate the direction cosines of the ³¹P couplings with the X-ray crystallographic data.