The EPR spectra resulting from electrochemical reduction of the phosphaalkene (I) have been obtained in both liquid and frozen solutions. The spin densities have been obtained from the spectra of the phosphaalkene of normal isotopic composition and of phosphaalkenes substituted by 13C and 2D and compared with those obtained for the diphosphene radical anion (II-). Ab initio calculations have been carried out on the model radical anions [HP==CH2]-, [HP==C(H)--CH==CH2]-, [HP==C(H)C6H5]-, and [CH3P==PCH3]-; the calculated SOMO's agree with the experimental results and show that the phosphorus spin density is practically the same in the diphosphene anion as in the benzene-containing phosphaalkene anion.