Amorphous hydrogenated carbon (a-C:H) and amorphous deuterated carbon (a-C:D) films were prepared using plasma-enhanced chemical vapor deposition (PECVD) from CD4, H-2, CD4 and D-2 source gases. Fourteen different samples were prepared by changing the source gas ratios of CH4/H-2, CH4/D-2, CD4/H-2, and CD4/D-2. The concentrations of hydrogen (H) and deuterium (D) relative to carbon (C) in the films were determined by elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS). The sp(2)/(sp(2) + sp(3)) ratios of the films were analyzed by near-edge X-ray absorption fine structure (NEXAFS) measurements. Hardness and mass density of the films were measured using a nanoindenter and X-ray reflectivity (XRR), respectively. For all combinations of source gas, the H and D concentrations varied by only 4.0 at.%. For the CH4/D-2 source gas, the D concentration in the film increased from 0 at.% with the D-2/(CH4 + D-2) source gas combination to 11.2 at.% for the 80% D-2/(CH4 + D-2) source gas combination. The increase in D concentration exceeded the increase in total H and D concentration (3.4 at.%). For CH4/D-2 source gas, the H concentration decreased as the D concentration increased. For the CD4/H-2 source gas, we observed the opposite tendency. Additionally, an isotope effect between the a-C:H films and the a-C:D films was observed, with preferential incorporation of H over D. From the NEXAFS measurements, the sp(2)/(sp(2) + sp(3)) ratios in all of the samples were between 38.8% and 40.8%. A correlation between the sp(2)/(sp(2) + sp(3)) ratio and the H-2 or D-2 gas source ratio was not observed. The hardness and density of the films decreased when the H-2 or D-2 source gas ratio increased. Even though the H concentration in the a-C:H films was higher than the D concentration in the a-C:D films, the a-C:D films had lower hardness and mass density values. These findings suggest that information concerning the voids, nanostructures, sp(2)/sp(3) ratios and H concentrations of hydrogenated amorphous carbon films is crucial for evaluation of their mechanical properties. (C) 2012 Elsevier B.V. All rights reserved.