Mg-Ni thin films were grown using Pulsed Laser Deposition. In situ optical changes from shiny metallic to transparent states were observed for films deposited in vacuum and under an Ar/H-2 gas mixture (93/7%), respectively. Optical changes were also achieved by ex situ hydrogenation under hydrogen gas pressure of 15 bars at 200 degrees C. However, after ex situ hydrogenation, the optical transmittance of the Mg-based hydrogenated thin films did not exceed 25%. Such limitation was attributed to oxygen contamination, as deduced by High Resolution Transmission Electron Microscopy observations, showing the co-existence of both Mg-based and MgO phases for as-deposited films. A significant decrease in oxygen contamination was successfully achieved with the addition of carbon, leading to the preparation of (Mg-based)-C-x (x < 20%) thin films showing a faster and easier hydrogenation. (c) 2006 Elsevier B.V. All rights reserved.