The composition of nitrogen-doped hydrogenated amorphous carbon films grown in a magnetically confined rf plasma-enhanced chemical vapour deposition system has been determined by X-ray photoelectron spectroscopy and compared with that determined using a combination of elastic recoil detection analysis, Rutherford back-scattering and nuclear reaction analysis. The importance of nitrogen doping or 'incorporation' in hydrogenated amorphous carbon films is discussed in relation to the significant variation in the sp 2 -to-sp 3 ratio that takes place. At 7 at.% N in the (...) a-C : H matrix, a critical change in the microstructure is observed, which governs the resulting mechanical, optical and electronic properties. Finally, the correlation between the sp 2 and sp 3 fractions determined by a non-destructive method of obtaining the bond fractions and by electron-energy-loss spectroscopy is discussed, with a view to evaluating accurately the sp 2 fraction in a-C : H : N films. (shrink)