In this research, six low-lying triplet states of diphosphene (HPPH) and disphosphinylidene (PPH2) are systematically investigated starting from self-consistent field theory and proceeding to multireference coupled cluster methods using a wide range of basis sets. For each structure, the geometry, energy, dipole moment, harmonic vibrational frequencies, and infrared intensities are predicted. The triplet potential energy surface (PES) of P2H2 is presented, based on systematically extrapolated coupled cluster energies and accounting for core-valence correlation, zero-point vibrational energy, and diagonal Born-Oppenheimer effects. Both (3)A" pyramidal PPH2 and B-3 skewed HPPH are minima on the triplet PES and lie 27.4 +/- 0.3 and 32.4 +/- 0.3 keal mol(-1) above the global minimum structure closed-shell (1)A(g) trans-HPPH, respectively. The energy barrier for the isomerization reaction 1(3)B skewed HPPH - (3)A" pyramidal PPH2 is predicted to he 16.4 +/- 0.3 keal mol(-1). On this triplet PES, two equivalent B-1 skewed HPPH are converted via the 3B(n) trans-HPPH transition state with a barrier of 9.1 0.3 keal mol(-1) or via the B-3(2) cis-HPPH transition state with a barrier of 11.1 +/- 0.3 keal mol(-1). Moreover, the two equivalent (1)A" pyramidal PPH2 structures are connected through the (3)A(2) planar PPH2 transition state with a barrier of 18.6 +/- 0.3 keal mol(-1). The energy crossing of the singlet and triplet adiabatic PES is studied using Mukherjee multireference coupled cluster method with the cc-pVQZ basis set, which predicts that the B-3 skewed HPPH is L4 keal mol(-1) lower in energy than the corresponding (1)A skewed HPPH at the B-3 skewed HPPH optimized geometry.