The dihydrogen (DiH2) and the dihydride forms of the W(CO)3(PH3)2(H2) complex are studied by means of DFT (B3LYP) and ab initio (MP2, MP4(SDTQ), and CCSD(T)) calculations. The oxidative-addition process involving simple H-H breakup DiH2 → PBI) is found to be a very low energy process (ΔH‡ = 2.50 kcal/mol at the CCSD(T) level). An essentially zero energy barrier for the reverse reaction explains why this structure with both the hydride and phosphorus ligands equivalent is not that observed in low-temperature NMR experiments. A new structure is proposed for the dihydride form, PB2, which accounts for both the spectroscopic and thermodynamic experimental data. It can be described as a pentagonal-bipyramidal structure, with two axial carbonyl ligands and two equatorial hydrides separated by a phosphine ligand. The enthalpy difference between DiH2 and PB2 is computed to be equal to 1.29 kcal/mol (in favor of DiH2) at the CCSD(T) level, in good agreement with the experimental data on related complexes (1.2-1.5 kcal/mol).