The potential energy surfaces for the reactions of diborane with four Lewis bases, XH(n) (NH3, H2O, PH3, and H2S), have been calculated at the MP4/6-311+G(d,p)//MP2/6-31G(d,p) level with zero-point energy corrections. Two steps for the formation of H2B=XH(n-1) were studied. The first step is the formation pathway of a complex, H(3)BH(3)BXH(n), or an adduct, H3B:XH(n), and the second is 1,2- and 1,3-hydrogen elimination from the complex or the adduct. For the formation of the complex or the adduct, the reactions of diborane with NH3 and PH3 systems are more favorable than those with H2O and H2S systems. The energy barrier heights for this step are proportional to the values of the proton affinity of Lewis bases. The transition states of 1,2-hydrogen elimination from the adducts have high activation barriers (42-49 kcal/mol) for the above four systems. These energy barrier heights are proportional to the dissociation energies of the X-H bond in their adducts. The transition state of 1,2-hydrogen elimination for BH3PH3 system, which has a structure similar to those of the others, leads to a PH3 inversion complex (not the adduct) for the reactant side along the IRC pathway. On the other hand, 1,3-hydrogen elimination from the complexes have low-energy barriers except for the B2H6 + PH3 system. For B2H6 + PH3 system, the reaction through 1,2-hydrogen elimination of H2B(H-2)PH2BH3 is the lowest barrier pathway. At high temperatures, Gibbs free energy pathways for these systems were also discussed.