The radical anions of various isomers of phenyldiphosphaalkenes were generated by electrochemical and chemical reductions. The resulting EPR spectra in liquid and frozen solutions show that the spin delocalization on the phosphaalkene groups is dependent upon the nature of the isomer. The corresponding spin densities are compared with those predicted by DFT calculations on model diphosphaalkene anions. The role of the bridging group between the two PC bonds was investigated by studying the diphosphaalkene derivatives of diphenyl and furan.