Using highly correlated wave functions, the ground and the low lying excited states of the molecular NCO(-) and CNO(-) anions have been reinvestigated. The stability of the electronic ground state of the two isomers with respect to dissociation and to electron detachment has been checked along the isomerization pathway. The regions of stability of the excited electronic states have been analyzed and identified and it is shown that only the ground state is stable and the corresponding potential energy surface presents three equilibrium positions. The rovibronic spectroscopy of the X (1)Sigma(+) state of both NCO- and CNO- isomers has been determined by a variational approach leading to remarkable agreement with experimental data. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3483463]