AbstractA low‐temperature ESR study on electron‐capture radicals in X‐irradiated alkyl/ phenylphosphine chalcogenides is presented. In general, exposure of solid R(3‐n)PhnPX derivatives (R = alkyl, Ph = phenyl, X = S, Se,n= 1, 2) to X‐rays gave ESR spectra assigned to the corresponding radical anions, in which the unpaired electron is located in an antibonding P‐X σ* orbital. For two of the compounds studied (Me2PhPSe and MePh2PSe), however, no phosphoranyl radicals could be observed. For methylphenyl‐n‐propylphosphine selenide, a single‐crystal ESR analysis of the σ* radical anion is presented, giving detailed information on its electronic and geometric structure. It is found that the unpaired electron resides in a PSe anti‐bonding σ* orbital, whose direction almost parallels the parent PSe bond. Aspects of radiation damage mechanisms and phosphoranyl radical formation in phosphine c