Silicon-related films have gained much interest as protective coatings for transparent polymeric materials used for automotive components to improve fuel economy and reduce greenhouse gas emissions. This study aims at synthesizing transparent and hard SiOC(-H) films to improve the properties of polycarbonate. SiOC(-H) thin films were synthesized from a mixture of trimethylsilane (TrMS) and O 2 gases with various mixture ratios by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method and the characteristics of the films such as transparency, hardness and chemical bonding were investigated as a function of the mixture ratio. The transparency, hardness and chemical bonding were analyzed by ultraviolet-visible (UV-vis) spectroscopy, nanoindentation and Fourier transform infrared (FT-IR) spectroscopy, respectively. As the mixture ratio increased, an increase in hardness and a deterioration in transparency were observed, where the relative ratio of Si-(CH 3) x (x=1, 3) bonds to Si-O-Si-related bonds increased and the number of Si-O-Si bonding in the caged structure decreased. Among the sample films prepared, the film synthesized at a partial pressure ratio of TrMS gas of 60% showed an optical transparency of nearly 100% and a hardness of 6.5GPa, which is equivalent to the hardness of conventional soda-lime glass. © 2011 Elsevier B.V.