Decarboxylation is normally a dissociative process, commonly catalyzed by proton or enzymes. The decarboxylation mechanism of pyrrole-2-carboxylic acid involves the addition of water to the carboxyl group, and the C-C bond cleavage leading to the protonated carbonic acid. The direct decarboxylation and decarboxylation aided with water were also investigated to consider the functions of proton and water. Our calculations with GAUSSIAN 03 package show that, with the assistance of H3O+, the potential energy of the C-C rupture decreases significantly to 9.77 kcal/mol, and the total energy barrier decreases to 33.99 kcal/mol. (C) 2010 Elsevier B.V. All rights reserved.