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--->[CH3-O-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. (C) 2004 American Institute Of Physics.