Composite separation membranes of poly(dimethylsiloxane) (PDMS) containing well-dispersed silicalite particles of 50 nm were successfully prepared and applied to the preferential pervaporation of acetic acid over water. The nanocomposite membranes showed improvement on both separation factor and permeation flux for the pervaporation process, as compared to plain PDMS membranes and composite membranes containing silicalite particles of 5 m. The improvement can be attributed to higher readily accessible specific surface area and higher sorption selectivity toward acetic acid of the nano-size silicalite particles. Improvement on membrane thermal stability was also acquired through incorporation of nano-size silicalite particles.