Determination of the electronic structure of mass-selected transient molecular ions which can be considered as building blocks of biomolecules. Ion yields following X-ray absorption of the cationic series NH y + ( y = 0–3) were measured to identify the characteristic absorption resonances in the energy range of the atomic nitrogen K-edge. Significant changes in the position of the absorption resonances were observed depending on the number of hydrogen atoms bound to the central nitrogen atom. Configuration interaction (CI) calculations were performed to obtain line assignments in the frame of molecular group theory. To validate the calculations, our assignment for the atomic cation N + , measured as a reference, was compared with published theoretical and experimental data.