The reactivity of hydrazine in the presence of diborane has been investigated using ab initio quantum chemical computations (MP2 and CCSD(T) methods with the aug-cc-pVTZ basis set). Portions of the relevant potential energy surface were constructed to probe the formation mechanism of the hydrazine diborane (BH₃BH₃NH₂NH₂) and hydrazine bisborane (BH₃NH₂NH₂BH₃). The differences between both adducts are established. The release of hydrogen molecules from hydrazine bisborane adducts has also been characterized. Our results suggest that the BH₃NH₂NH₂BH₃ adduct, which has been prepared experimentally, is formed from the starting reactants hydrazine + diborane. The observed adduct is produced by a transfer of a BH₃ group from BH₃BH₃NH₂NH₂ rather than by the direct attachment of a separate BH₃ group, generated by predissociation of diborane, to BH₃NH₂NH₂.