Silicon incorporation into DLC films prepared by plasma enhanced chemical vapour deposition (PECVD) was studied by a combination of surface analysis methods and nanomechanical measurements; namely XPS, Raman spectroscopy and nanoindentation. Addition of silicon into the films leads to an increase in the sp(3) contribution, as measured from XPS analysis, and a decrease in the Raman band intensity ratio I-G/I-G. These changes are consistent with an evolving C-C bond network. The mechanical properties were first studied as a function of film thickness and indentation depth to assess the effect of substrate proximity. Silicon incorporation produces films with lower hardness and Young's modulus. It is suggested that, for such a PECVD process, the weakening of the mechanical properties is caused by the increased hydrogen content in the doped films, as shown by the increased Raman background slope, These tendencies are attributable to the development of polymer-like chains, which weakens the inter-molecular structure ofthe films. (C) 2001 Elsevier Science B.V. All rights reserved.