The reaction mechanism of the acid-catalyzed transesterification reaction of butyric acid triglyceride was studied employing the Density Functional Theory (DFT). Three different pathways were considered for the three steps of the transesterification. Each step of the reaction proceeds through a concerted mechanism with one single transition state and without the formation of a tetrahedral intermediate. For transition states at each step of the reaction, bond formation was observed between the carbonyl carbon and the oxygen of the incoming nucleophilic and bond breaking between this carbon and the oxygen of the leaving group. This occurs concomitantly with proton transfer from the nucleophilic oxygen to the oxygen of the leaving group. Bond order analysis revealed the degree of bond breaking and formation in transition states which are stabilized through interactions with both attacking and leaving groups. DFT results show that the transesterification of the center ester bond followed by the subsequent transesterification of the outside ester bonds is most probably the pathway through which the reaction proceeds. (C) 2011 Elsevier Ltd. All rights reserved.