A systematic study of the singlet and triplet PXH(2) potential energy surfaces, X ranging from nitrogen to bismuth, has been undertaken through ab initio methods. Geometries were optimized at the Hartree-Fock level, and single-point energies taking into account the electron correlation effects were calculated. Effective core potentials, including the main relativistic effects, have been used for the heaviest X atoms, i.e. arsenic, antimony, and bismuth. General trends in the structural and energetic properties of the various isomers, when going down group 15, are given. The possibility of forming P=Sb and P=Bi double bonds is confirmed, as the trans HPSbH and HPBiH isomers are found to be global minima on the singlet potential energy surfaces. The H2PSb and H2PBi isomers in their triplet states appear to be lower in energy than the corresponding trans (closed-shell) isomers, by 0.8 and 7.5 kcal/mol, respectively. However, the energy barriers are large enough to prevent any isomerization of the HP=BiH species to H2PBi singlet or triplet species. On the other hand, the thermodynamic stability of the P=X double bond is predicted to decrease when X varies from nitrogen to bismuth, which can explain the difficulties encountered by the experimentalists to synthesize compounds containing P=Sb and P=Bi double bonds.