In this study, SiOCH films for gas separation were prepared on a mixed cellulose ester (MCE) substrate by means of plasma-enhanced chemical vapor deposition (PECVD) using octamethylcyclotetrasiloxane (OMCTS) as a precursor. The RF power was the major variable in adjusting the pore structure of the SiOCH film. The FTIR analysis showed that there was a breakup of the Si-O-Si cyclic ring with increasing RF power. According to the positron annihilation lifetime spectroscopy (PALS) characterization, the SiOCH films had two different pore sizes as indicated by the following: tau(3) forming the matrix structure and tau(4) representing big cages. The range of the pore radius was 3.52-3.78 angstrom for the matrix structure and 6.31-7.47 angstrom for big cages. With the RF power increasing to 150 W from 50 W, the SiOCH films showed a higher crosslinking, resulting in a decrease in gas permeability (for example, 40-14 GPU for O(2)) and an increase of O(2)/N(2) selectivity from 1.67 to 2.31. At the same time, based on FTIR and PALS analyses, the OMCTS cyclic ring would open up and then form a small amount of larger pores (tau(4)). Based on the FTIR analysis, all the SiOCH films prepared at different RE power showed a very high organic character that indicated values of I(Si-CH(3))/I(Si-O-Si) higher than 0.65. (C) 2010 Elsevier B.V. All rights reserved.