Abstract: One of the most important transformations of glycidic esters is their decarboxylation to give rise to aldehydes and ketones. A proposed mechanism for the decomposition of the ethyl 3-phenyl glycidate indicates that the ethyl side of the ester is eliminated as ethylene through a concerted six-membered cyclic transition state, and the unstable intermediate glycidic acid rapidly decarboxylates to give the corresponding aldehyde. Two possible pathways for the glycidic acid decarboxylation were proposed, one of them via a five-membered cyclic transition state and the other one via a four-membered cyclic transition state(Chuchani, G.; Tosta, M.; Rotinov, A.; Herize, A. J. Phys. Org. Chem. 2004, 17, 694). An accurate experimental description of the glycidic acid decarboxylation is a challenging task, since the glycidic acids decompose even at room temperature. Therefore, the goal of this work is investigated the path more favored by decarboxylation. Theoretical calculations were carried out in order to probe this proposed mechanism. Geometries of the different reactants, products and intermediates were optimized at different levels of calculation. The vibrational frequencies were calculated at the same level. The results were compared with those from experiments. Theoretical calculations indicate that the glycidic acid decarboxylation occurs via a five-membered cyclic transition state.