Density functional theory (DFT)/BMK and CBS-QB3 ab initio calculations have been carried out to study the structures and energetics of unimolecular decomposition reactions of isopropyl butanoate (IPB, C(3)H(7)C(O)OCH(CH(3))(2)) as a model biofuel. The results show a good performance of the BMK method. Among seven different dissociation channels of IPB, formation of butanoic acid and propene via a six-membered ring transition state is the most favorable reaction. On the other hand, formation of lower esters is hindered by high-energy barriers and unlikely occurs except at elevated temperatures. Simple bond scission costs less energy than lower ester formation. A comparison with methyl and ethyl esters indicates faster decomposition of IPB. The changes in bond lengths along minimum energy paths are discussed.