Ab initio MO calculations at the QCISD(T)/6-31G**//MP2/6-31G** + ZPE level on the XX'CP2 isomers (X = H, F and Cl) show that the energies of the isomeric species are often reordered following halogen substitution. The phosphorus-halogen moiety behaves as the main stabilizing factor on the rest of the molecular structure. This is particularly true when X = F is involved. In the dihalogenated series phosphanylphosphaalkyne (4) becomes the most stable species followed by diphosphaallene (3) and 1H-diphosphirene (6). The effect on the geometry following halogenation is larger if substitution takes place at carbon than at phosphorus. The behaviour of the P species is found to be quite different from, if not opposite to, that of their nitrogen analogues. Finally, the infrared spectra of the most stable isomers are also predicted.