A dimer radical possessing two anisotropically equivalent phosphorus atoms has been conclusively identified inγ-irradiated trimethyl phosphite, its E.S.R. spectrum consisting basically of four lines due to the large second-order splitting of the central component. In the powder spectrum of theγ-irradiated glass the central features are obscured by overlapping signals from other radicals. However, the downfield (1, 0) feature was clearly revealed by suppressing the formation of the interfering P(OCH3)2radical through the inclusion of methyl bromide in the sample. The E.S.R. spectrum of aγ-irradiated single crystal is very well resolved and three complete sets of lines can be assigned unequivocally to chemically non-equivalent dimers. The spectra of two of the dimers undergo site splitting such that the symmetry elements for each pair of magnetically distinguishable sites are related. In addition, each of the lines belonging to these two dimer spectra is flanked by a pair of ‘spin-flip’ satellites at high microwave power. Chemical and structural evidence strongly suggests that the dimer is the centrosymmetric cation (CH3O)3P-P(OCH3)3/+, the unpaired electron occupying aσ* orbital built from the 3sand 3porbitals of the two phosphorus atoms with ap/shybridization ratio of approximately 2. The chemical non-equivalence of the dimers may result from slight differences in this hybridization ratio.