Exposure of dilute solutions of trimethylphosphine sulfide (Me3PS) in methanal (CD3OD)-water glasses to ionizing radiation at 77 K gave the parent radical anion Me3PS-, identified by its EPR spectrum. This is shown to have a trigonal bipyramidal structure characteristic of phosphoranyl radicals. Similar features were obtained for the pure compound at 77 K, with resolved proton splitting on the parallel features from one CH3 group. Doublet features assigned to the parent cations, Me3P-(S) over dot(+) were observed for solutions in freon (CFCl3) and also in the pure compound. The latter also gave features assigned to H-2(C) over dot P(Me-2)S and/or H-2(C) over dot P(Me)(2)SH+ radicals. On annealing to ca. 140 K, doublet features with a considerably reduced P-31 hyperfine splitting, assigned to the phosphoryl radical Me-2(P) over dot S, were revealed for the pure compound. At 77 K, these features were further split into 1:2:1 triplets, apparently from only two equivalent protons. However, at ca. 140 K, septets from all six protons were obtained with almost the same separation between the outmost features. Also, the original P-31 parallel and perpendicular features gave way to features that had converged, although some anisotropy was still seen. At still higher temperatures, an isotropic doublet of septets was observed, prior to loss of the features. This is assigned to freely rotating Me-2(P) over dot S radicals. Possible mechanisms for the formation of these Me-2(P) over dot S radicals are discussed. The only reasonable one that we can discover is electron return to give electronically excited (Me3PS)* molecules, which dissociate to give (C) over dot H-3 and Me-2(P) over dot S radicals. The former radicals ((C) over dot H-3) were not detected, but they may well react to give methane and the H-2(C) over dot P(Me)(2)S radical. The reversible change from 3 to 7 lines from the two methyl groups in Me-2(P) over dot S radicals is discussed in terms of hindered rotation for the two methyl groups.