Cyclic voltammetry shows that monophosphaallene ArP=C=C(C6H5)(2) (where Ar = (C6H2Bu3)-Bu-t-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific C-13 enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HP=C=CH2)(-.) and for the monophosphaallylic radical (HP.-CH=CH2) <----> (HP=CH-(CH2)-C-.). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.