Neutral and ionic pathways for aci-nitromethane decomposition were studied in the gas phase, and the Gibbs free energy surfaces were constructed at 473 K. The pathways studied were based on proposed mechanisms of NOx reduction by acetaldehyde over BaNaY zeolites. Density functional theory at the B3LYP/6-311++G(d,p) level of theory was used, and 29 stable intermediates and 39 transition states were identified and quantified. Plausible pathways involving unimolecular decomposition or NO2 addition were both explored. The rate constants for all elementary steps were estimated using transition state theory, and kinetic modeling was carried out to identify the dominant reaction channel. For the ionic routes, one of the. NO2 addition pathways dominated at typical NO2 concentrations. For the neutral routes, one of the unimolecular decomposition routes was dominant. A solvation model was then included to mimic the environment of the BaNaY zeolite catalyst in a simplified manner. While inclusion of solvation effects stabilized the ionic species significantly, the dominant reaction channels in both the neutral and ionic systems were not altered. Our results show that the addition of NO2 facilitates the decomposition of aci-anion nitromethane.