Ab initio molecular orbital theory has been used to investigate the gas phase decarboxylations of the beta-keto carboxylic acids XCOCH2COOH (X = H, OH, and CH3). Structures for stationary points representing reactants, transition states, and products on the potential energy surface have been optimized and characterized at the MP2(Full)/6-31G* level of theory. Based on these, the single-point calculations were performed at the MP4SDTQ(FC)/6-311++Ga**//MP2(Full)/6-31G** level of theory. The reaction pathways connecting the transition structures and the corresponding equilibrium structures were followed by the intrinsic reaction coordinate (IRC) procedure. For each decarboxylation, channels via the four- and six-membered ring transition structures were both considered. In all cases. the latter are found to be energetically favored over the former. The predicted decarboxylation barriers in the gas phase are 23.8. 23.3, and 28.5 kcal/mol for 3-oxopropanoic acid, acetoacetic acid, and malonic acid, respectively. The computed value of 28.5 kcal/mol is in good agreement with the corresponding experimental results of malonic acid obtained in solutions of various solvents. Self-consistent reaction field (SCRF) calculations based on Onsager's solvent cavity model were performed to assess the solvent effect on the changes in the geometries of the reactants and transition structures. The energy barriers in going from the gas phase to the medium were also explored. Results show that the changes are marginal. No experimental data of decarboxylation are available for 3-oxopropanoic acid and acetoacetic acid for comparison. Using the computed relative energies, moment of inertia, and vibrational frequencies, transition state theory (TST) rate constants were calculated for the decarboxylations in the gas phase.