The doublet potential energy surfaces involved in the decomposition of the nitromethane radical anion (CH3NO2−) have been studied by using the multistate extension of the multiconfigurational second-order perturbation method (MS-CASPT2) in conjunction with large atomic natural orbital-type basis sets. A very low energy barrier is found for the decomposition reaction: CH3NO2−→[CH3NO2]−→CH3+NO2−. No evidence has been obtained on the existence of an isomerization channel leading to the initial formation of the methylnitrite anion (CH3ONO−) which, in a subsequent reaction, would yield nitric oxide (NO). In contrast, it is suggested that NO is formed through the bimolecular reaction: CH3+NO2−→[CH3O-N-O]−→CH3O−+NO. In particular, the CASSCF/MS-CASPT2 results indicate that the methylnitrite radical anion CH3ONO− does not represent a minimum energy structure, as concluded by using density functional theory (DFT) methodologies. The inverse symmetry breaking effect present in DFT is demonstrated to be responsible for such erroneous prediction.