The equilibrium structures of P,H, isomers and the associated isomerization transition states have been investigated systematically starting from self-consistent-field theory and proceeding to coupled cluster methods using a wide range of basis sets. For each structure, the geometry, energy, dipole moment, harmonic vibrational frequencies, and infrared intensities have been predicted. The global minimum has been confirmed to be planar trans-HPPH diphosphene, lying 3.2 kcal mol(-1) below cis-HPPH with the aug-cc-pVQZ CCSD(T) method upon inclusion of zero-point vibrational energy corrections. Diphosphinylidene, which has the connectivity PPH2 and C-2 nu symmetry, lies 25.2 kcal mol(-1) above the global minimum. The trans-cis isomerization reaction occurs via internal rotation with a barrier of 35.2 kcal mol(-1) using the cc-pVQZ Mk-MRCCSD (2e/2MO) method. This transition state exhibits multireference character and consequently properties were evaluated using CASSCF, MRC1, CASPT2, and Mk-MRCCSD methods with various basis sets. At the aug-cc-pVQZ CCSD(T) level, the transition state for the isomerization reaction between trans-HPPH and diphosphinylidene (planar PPH2) is predicted to be nonplanar with a torsional angle of 101.1 degrees. The corresponding barrier is estimated to be 48.2 kcal mol(-1).