This investigation was performed to find if the nanometer SiO2 added in the membranes can improve the pervaperation performance of the membranes. Acrylic acid (AA) and acrylonitrile (AN) were synthesized by solution polymerization with and without nanometer SiO2. The copolymer solution was made into main body of the membranes, then composited with the polyvinyl alcohol (PVA) acetal membranes, to make the three-layer sandwich composite pervaporation membranes. The structure and the performance of the membranes were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry (TG), dynamic themomechanical analysis apparatus (DMA) and mechanical property testing. Pervaporation experiments were carried out using these membranes to separate the mixtures of methanol/water over the complete concentration range 70-98%, and results showed that the selectivity of the membranes with nanometer SiO2 had notable improvement. For the 98% mixture at 60 degrees C, the separate factor is up to 1458, which is improved more than 10 times compared to the membranes without nanometer SiO2, the permeate flux is up to 325 g/(m(2)h). For the 70% mixture at 70 degrees C, the separate factor arrived at 12, the permeate flux is up to 7097 g/(m(2)h), which is improved more than 14 times compared to membranes without nanometer SiO2. It was concluded that the pervaperation performance of the membranes can improve greatly by nanometer SiO2. (c) 2008 Elsevier B.V. All rights reserved.