Theoretical models and ab initio Hartree-Fock wave functions are used to investigate the N Is core level binding energies of N-containing calcined carbonaceous materials. Comparison of calculated and experimental values for a series of test molecules reveals that the N Is core level shift from one compound to another is mainly originated by initial state effects. This permits a systematic study of different situations and allows establishment that three different types of nonoxidized N atoms can be present in these materials. These are "pyridinic", "pyrrolic", and "graphitic" nitrogen with binding energies of approximate to 399.0, approximate to 400.3, and approximate to 401-403 eV, respectively. This assignment is in very good agreement with a recent experimental X-ray photoelectron spectra on petroleum cokes and demonstrates, for the first time, that it is possible for N to exhibit rather large core level Is energies without requiring the presence of any charge transfer from N-oxide groups. Theoretical reasons for such a behavior are also given.