Amorphous carbon nitride films, prepared using a dc facing-target reactive sputtering system, were annealed at temperatures up to 650 degrees C for I It in vacuum. The effects of heat treatment on the films, i.e. changes in the composition and structure, were investigated. It was found that annealing at temperatures ranging from 300 to 650 degrees C, results in the N content decreasing from similar to 33 at.% in the as-deposited films to similar to 5 at.%. The loss of N, especially those bonded to sp(3)C, causes the rearrangement of the film's microstructure, and the dual effects of the thermal annealing are quite noticeable: (1) annealing destroys most graphite-like structures, and more non-aromatic sp(2)C components and C equivalent to N terminal structures are formed at higher annealing temperatures, contributing to a looser film's structure. (2) Annealing makes the remaining aromatic sp(2)C structure become more order. The results also reveal that N atoms bonded to sp(3)C are easily removed with the increasing temperature compared to those bonded to sp(2)C, which indicates that N-sp(2)C bonds had a higher thermal stability than N-sp(3)C. (c) 2006 Elsevier B.V. All rights reserved.