Despite having many potential applications, polypropylene (PP) has limitations that may make its application difficult when there is a requirement for high transparency and low-gas permeability. In this context, studies have been reported about the usage of amorphous hydrogenated carbon thin films to act as a functional barrier in polymer packaging. Therefore, this current work evaluated the modification on the surface-clarified polypropylene (cPP) by oxygen plasma to improve the adhesion of amorphous hydrogenated carbon films (a-C:H) deposited by plasma-enhanced chemical vapor deposition. However, some applications of these films are limited by the presence of microdefects and low adhesion when applied onto polymeric substrates. So, the surface of cPP was treated with oxygen plasma, and the adhesion between the a-C:H films and cPP was evaluated. Contact angle goniometry, infrared spectroscopy, and atomic force microscopy were used to analyze chemical and morphological changes, respectively. Tape test and scanning electron microscopy were used to evaluate the adhesion of a-C:H films deposited on a cPP surface. The results showed that the pretreatment increased the adhesion between the cPP and amorphous hydrogenated carbon, which is crucial for the application of these films in packaging with high-barrier properties. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers