Silica hybrid membranes have been developed for use as reverse osmosis (RO) membranes by using a counter-diffusion chemical vapor deposition (CVD) method. A silica source (phenyltrimethoxysilane; PhTMOS) and O-3 were provided at opposite sides of a porous alumina substrate at 300 degrees C for 90 min. The RO permeation test was conducted for 100 mg L-1 NaCl at 3.0 MPa. The highest NaCl rejection was 94.2% for a total flux of 1.7 kg m(-2) h(-1). The module length is an important factor in obtaining highly selective RO membranes. The short module (6 cm) was better because of the higher O-3 concentration in the module. The decomposition conditions of phenyl groups on the silica surface are discussed for the hydrolysis powder of PhTMOS. According to FT-IR measurements, phenyl groups remained on the silica surface after O-3 treatment for 90 min at 300 degrees C, whereas the number of silanol groups decreased by approximately 30% upon O-3 treatment at 300 degrees C. The high Na+ rejection can be explained by the reduction of the silanol groups in the membrane.