Amorphous hydrogenated carbon films with different nitrogen content (a-C : H : N) were deposited (thickness 2.65-3.01 m) by plasma-activated chemical vapour deposition of acetylene#nitrogen (0-62 vol%) onto glass/Si substrates at a deposition temperature of &523 K and a negative bias voltage &500 V. The fundamental optical absorption edge was studied by optical transmittance measurements. With increasing nitrogen content (0-20 at%) in the films the band gap and Urbach tail width decreased. Thick films deposited without nitrogen had high compressive stress ( &3 GPa). Introduction of nitrogen in the precursor gas was effective in reducing the stress to less than 1 GPa with improved adhesion of the film on the substrate. The stresses in the films were determined by a non-destructive optical technique (from the optical absorption band tail) so that the substrate effect on the stress measurement prevalent in the indentation technique could be eliminated. Raman, photoluminescence and FTIR spectra were analysed to ascertain the quality of the films. Studies on the temperature dependence of the electrical conductivity of the films indicated variable range hopping conduction to be predominant at low temperature.