An SiOxCyHz/polycarbonate (PC) composite membrane was prepared via plasma deposition of tetraethoxysilane (TEOS) monomer on the surface of the PC substrate for the purpose of applying it in the pervaporation of an 80 wt.% aqueous tetrafluoropropanol (TFP) solution. The effects of plasma power and deposition time on the chemical and physical characteristics of the composite membrane were investigated with Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). To correlate the free volume variation in the plasma deposition layer of the SiOxCyHz/PC composite membrane at different plasma treatment conditions, positron annihilation spectroscopy (PAS) experiments coupled to a variable monoenergy slow positron beam was performed. Under a condition of high-applied power, an SiOxCyHz, layer with low porosity could be produced, and it showed good water selectivity in the pervaporation of an 80 wt.% aqueous tetrafluoropropanol (TFP) solution. The best pervaporation performance was obtained with the SiOxCyHz/PC composite membrane at an applied power of 150 W and a deposition time of 30 min as the plasma treatment conditions, giving a permeation rate of 346 g/(m(2) h) and a water concentration in the permeate of 99.3 wt.%. (c) 2009 Elsevier B.V. All rights reserved.