Two radiogenic radicals trapped in a single crystal of 1-[2,4,6-tri-tert-butylphenyl]-2-phenylphosphaethene have been studied by EPR and have been identified, from their P-31 hyperfine tensors, as being phosphoniumyl radical cations. The spectra modifications caused by C-13 or D-2 enrichment of the phosphaalkene moiety show that these species result from an intramolecular cyclization which can lead to two possible conformations of the radical. The experimental P-31, C-13 and H-1 hyperfine tensors are compared with those predicted by ab initio calculations on model phosphoniumyl radical cations. These calculations show that these interactions are very sensitive to the geometry of the radical and that their measurement can yield precise structural information.