The gas phase reaction mechanism of the base-catalyzed transesterification of butyric and pentylic acid monoglycerides was studied employing Density Functional Theory. Solvent effects were included by means of the integral equation formalism for the polarizable continuum model (IEFPCM). For the ethoxide ion attack, in the absence of a counter ion and regardless of solvent effects, the well-known stepwise mechanism with a tetrahedral intermediate connecting two transition states was revealed. With and without solvation the results reveal the formation of the tetrahedral intermediate as the rate determining step with an activation free energy of 5.5 kcal mol(-1). In the presence of a counter-ion, e.g., in the case of EtONa(K) and MeONa(K) attacks in the gas phase, the reaction proceeds through a concerted mechanism with one single transition state (TS) and without the formation of a tetrahedral intermediate. The inclusion of solvent effects reveals that the transesterification reaction proceeds through a stepwise mechanism for the EtOK attack only, where the formation of the tetrahedral intermediate is the rate determining step with an activation free energy of 7.8 kcal mol(-1). The activation energies obtained were basically independent of the length of the side chain. (C) 2012 Elsevier Ltd. All rights reserved.