Plasma polymerized membranes were prepared from octamethyltrisiloxane and were applied to the pervaporation of ethanol-water mixture. The structure of the plasma polymer was analyzed by IR, XPS and solid-state NMR. When the parameter W/FM (W, power input; F, monomer flow rate; M monomer molecular weight) was small, the plasma polymer considerably preserved the monomeric structure. With increasing W/FM, both the extent of the cross-linked structure and the amount of oxygen incorporated in the polymer increased. In the relationship between the C/Si ratio analyzed by XPS measurements and W/FM, there was a minimum. The correlations between the pervaporation performances and W/FM as well as between the deposition rates and W/FM showed single trends. Although permeation rates decreased monotonously with the increase of W/FM, the separation coefficients showed a maximum. The value of W/FM at which the separation coefficient was maximum corresponded to the value at which the C/Si ratio was minimum. The separation coefficient and permeation rate obtained were 5.1 and 6.2 kg/m2 h, respectively. The high permeation rate was characteristic for the plasma polymerized membranes.