Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)–, of ab initio calculations performed on the model radical anion (HPCPH)– at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)–. The structure of the radical anion is compared with that of the neutral molecule.