Experimentally, gas-phase reactions between diborane (B2H6) and carbon monoxide (CO) produce borane carbonyl (BH3CO) and a less volatile material. To elucidate the unknown part of the products, we investigated the reactions of the B2H6:CO = 1:1 system using ab initio calculations at the MP2/6-311++g** level. Within the energy range of 120 kcal/mol, we located 41 minimum and 35 transition states on the potential energy surface. Among the intermediates and products, BH2OBHCH3 is thermodynamically most stable. Its formation releases energy of 64.59 kcal/mol and requires an activation energy of 36.20 kcal/mol at the rate-limiting step. On the contrary, the initially formed BH3CO + BH3 absorbs 14.75 kcal/mol of energy and requires 19.53 kcal/mol of activation energy. The results indicate that B2H6 can act as a reducing agent to hydrogenate CO at thermal equilibrium, whereas the formation of dative complex BH3CO is kinetically favored. Theoretically, several products containing boron, hydrogen, carbon, and oxygen can be isolated.