The purpose of this study is the synthesis of silica-like membranes deposited on the inner surface of an asymmetric porous alumina tube for gas separation. The deposition technique used in this work is the plasma-enhanced chemical vapor deposition (PECVD). The reactor, specially setup at the laboratory for this purpose, is used here for the deposition of a-SiCxOy:H thin films on the inner surface of tubular substrates at low temperature and involves the reaction between an organosilicon monomer vapor diluted in a mixture of oxygen or nitrogen protoxide and argon. The glow discharge is transported up to the surface and monomer fragmentation occurs close to it. The substrate tube can translate along its main axis in order to obtain a homogeneous layer along several centimeters. The morphology and chemical composition of the layers have been investigated as a function of the plasma discharge (gas flow rates, gas mixture composition, and plasma power) by scanning electron microscopy and infrared spectroscopy, whereas mass spectrometry helped to choose the conditions to form an organic/inorganic membrane. First results show that the deposition rate is very high (several micrometers per minute) and leads to a dense homogeneous organic-like material. Nevertheless, this one presents a good thermal stability up to 500 degreesC under an inert atmosphere. Currently, conditions to obtain much slower deposition rates and inorganic layer are optimized to obtain silica-like membranes for hydrogen separation. (C) 2003 Elsevier Science B.V. All rights reserved.