A series of samples were prepared by reactive laser ablation of a graphite target in nitrogen environment. The CNx films were annealed from room temperature to 900 degreesC, and in situ analyzed by electron energy loss spectroscopy (EELS), Auger electron spectroscopy, X-ray photoelectron spectroscopy and temperature desorption spectroscopy (TDS). A [N]/[C] ratio of 0.43 was determined and decreases notoriously as the annealing temperature increases. The peak shapes and intensities in the N-1s and C-1s regions are similar to those reported by other research groups. Small satellite peaks were observed in the N-1s and C-1s regions, corresponding to first- and second-order inelastic energy losses of the main peaks. These losses were detected also by EELS with an energy of 4 eV and assigned to the pi-pi (*) plasmon. This resonance is distinctive of non-localized electrons in an sp(2)-hybridized carbon orbital. Thereafter, the main N-1s and C-1s peaks could be assigned to atoms in a six- and five-member rings. Moreover, the films were easily degraded by the thermal treatment and the main desorption product determined by TDS was found to be C2N2. We concluded that the most probable structure is one where N atoms dope graphite-like carbon layers by substitution, causing three-dimensional polymerization reactions, with chemical states similar to Upilex, Kapton and other polyimides.