This present study aims to determine the hydrogen influence on the electrical gap of diamond-like carbon (DLC) film. DLC thin layers were deposited on silicon wafer by plasma enhanced chemical vapour deposition (PECVD). With this method we obtain a a-C:H film structure. To understand the mechanism of deposition, local structure and hydrogen effect in DLC we replace our methane plasma source by deuterated methane one. In this article, hydrogen rate is obtained by elastic recoil detection analysis (ERDA), electronic and bulk density is performed with X-ray reflectometry (XRR) and the determination of electrical gap is carried out using ultraviolet-visible absorption spectrometry. A specific attention is carried about the self-bias voltage and time variations to get different DLC and DDLC film compositions and also different electrical properties. We found that including deuterium in carbon increases the Tauc energy while keeping the same density. A comparison between these two sorts of film is expected to improve our understanding of the hydrogen role in the DLC films. (C) 2007 Published by Elsevier B.V.