The isomerization reaction of 2-butenedioic acid in gas phase has been studied using Hartree-Fock (HF), Meller-Plesset to 2nd order (MP2) and BLYP, B3LYP Density Functional Theory (DFT) levels of approximation. Potential energy surfaces (PES) of the singlet and triplet states of the molecule are constructed in order to corroborate the hypothesis that the reaction proceeds by a non-adiabatic path. A total of ten stationary points (eight singlet and two triplet) were characterized as energy minima or saddle points. Thermochemical analysis of the triplet conformers and singlet most stable structures yields a value of the AH0 of activation for isomerization that is in good agreement with the expected results.