In this study, further insight into the mass spectrum assignment of NbN- is provided using photoelectron imaging spectroscopy and nominal-mass counterpart analysis. Density functional theory is employed to determine ground state structures and detachment (adiabatic and vertical) energies of NbN0/- and its nominal-mass counterpart NbCH20/-. The calculated detachment energies of NbN- and NbCH2- are then compared with the experimental one which was measured for the mass-selected NbN- by the velocity-map imaging technique. The comparison with experiment shows good agreement with the value calculated for NbN- whereas the calculated value for NbCH2- deviates by >0.25 eV from the experimental one. It indicates that the peak observed in the mass spectrum belongs to NbN-. (C) 2014 Elsevier B.V. All rights reserved.