Thermal decomposition behavior of carbon nitride films prepared via nitrogen ion implantation into graphite in a special mass-separated low-energy ion beam system was investigated using in situ X-ray photoelectron spectrometry (XPS), residual gas analysis and secondary ion mass spectrometry. The XPS N1s line of the CNx films could be deconvoluted into three components, which are denoted as N(I), N(II) and NO(III) states with binding energies of similar to 398, 400 and 402 eV, respectively. A dramatic evolution of the N1s line during annealing of a room-temperature-implanted CNx film was revealed. Upon heating, the two main components NO) and N(H) split. and the NO) and NOW components attenuated rapidly with increase in temperature until they vanished at 1000 degreesC, leaving only the N(II) component. The N(II) component exhibited no significant change during heating up to 800 degreesC but decreased upon further heating to a higher temperature. These results indicate that the N(II) state is more thermally stable than the NO) state. The assignment of the bonding states between carbon and nitrogen atoms is discussed according to these results and the literature. The desorbed species upon heating of CNx films were monitored using a quadrupole mass spectrometer. The results showed that heating of CNx films led to desorption of N-2 and C2N2 molecules. Above 1000 degreesC, highly significant emission of CN- ions was observed as a result of dissociative negative ionization of desorbing C2N2 molecules.