The effect of nitrogen addition on the structure and nanomechanical properties of tetrahedral amorphous carbon, t alpha -C, has been studied. The t alpha -C films were grown on Al2O3-TiC substrates by reactive pulsed KrF excimer laser ablation of graphite targets at a laser fluence of 10 J cm(-2). Nitrogen contents up to 19 at.% were obtained by increasing the nitrogen partial pressure, P-N2 to 75 mTorr. The sp(3) content in the t alpha -C film as determined by analysis of the XPS C Is core level spectra had a value of about 76%. Incorporation of a small amount of nitrogen, 2 at.%, reduces the clustering of the sp(2) phase and improves the nanomechanical properties of the t alpha -C films, whilst for higher nitrogen concentrations the carbon bonding changes progressively from sp(3) to sp(2). Quantitative analysis of the Raman spectra indicated that incorporation of nitrogen greater than 2 at.% induced a progressive long-range order in the amorphous carbon and an increase in the size Of sp(2) graphitic clusters. Additionally, Raman spectroscopy established the presence of C equivalent toN bonds at high P-N2. To elucidate the influence of the substrate on the measurement of the nanomechanical properties of thin film a continuous measure of hardness and modulus as a function of depth was performed. Both the hardness and Young's modulus were significantly reduced from 56 and 573 GPa for CN0.02 to 2 and 44 GPa for CN0.19 at a contact depth of 25 nm. The deterioration of the nanomechanical properties with N incorporation is consistent with the spectroscopic results, which indicate a structural transformation from an amorphous structure consisting predominately of spl C bonds to an spl graphitic-like phase.