The mechanism of H₂ production from a Mo(I) complex with terpyridine and phosphine ancillary ligands, [Mo(NH₃)(^Phtbpy)(PPh₂Me)₂]⁺ was computationally investigated by DFT calculations. Several tentative pathways were tested and the most favourable one corresponds to a bimolecular mechanism that starts with the oxidative addition of the N−H bond in NH₃, yielding a 7‐coordinate Mo(III) species with a hydride and an amido ligand. In a second step two such intermediates join to form H₂ and the Mo(II) co‐product, [Mo(NH₂)(^Phtbpy)(PPh₂Me)₂]⁺. The first step has the highest barrier and its value (26.5 kcal/mol) is in agreement with mild reaction conditions, as experimentally observed (6 hours at 60 °C). The entire process occurs with spin change from S= in the initial complex to S=0 in the products. The same mechanism is also operative using water and methanol as ligands, the corresponding free energy barrier being ca. 7 kcal/mol lower than the one calculated for NH₃.