Amorphous hydrogenated carbon (a-C:H) exhibits outstanding properties such as high hardness, low mechanical wear and friction, high thermal conductivity, etc. These properties are irreversibly altered above 400 degrees C. Doping the a-C:H films with nitrogen revealed the possibility to continuously tune properties such as internal stress, hardness, electrical conductivity and surface energy. X-ray photoelectron spectroscopy has been used to analyze the chemical and structural modifications caused by the introduction of nitrogen into the films. An increased onset temperature of the thermal C-H bond decomposition in a-C:H films up to 600 degrees C (in vacuum) is observed. Hardness measurements on N-doped samples show an increase in thermal stability of the films, however, without ever reaching the hardness values obtained from the undoped a-C:H film.