Radical additions of tert-butoxyl radical and alkylthiyl radicals to 2-[(2,4,6-tri-tert-butyl)phenyl]-1-phosphaalkyne have been investigated by ESR. The addition of oxygen or sulfur centered radical leads to an intermediate phosphavinyl sigma-radical, which dimerizes to a 1,4-diphosphabutadiene, which undergoes a second radical addition to give a persistent pi radical species. These species exhibit couplings with two different phosphorus nuclei and two different pairs of meta hydrogens. Our assignments were supported by H-2 labeling and ab initio calculations.