Raman spectroscopy has been used to investigate the structural changes in thermally annealed diamond like carbon (a-C:H) and silicon modified diamond like carbon (a-C:H:Si) films prepared by plasma enhanced chemical vapour deposition (PECVD) using a 514.53 nm argon ion laser excitation. The changes in the Raman spectra of the films has been used to monitor structural modifications with increasing annealing temperature. The present investigation indicates that the rate of these structural modifications is dependent on both the annealing temperature and the negative self-bias voltage applied during the film deposition process for a fixed annealing time. (C) 1999 Elsevier Science B.V. All rights reserved.