Microstructures of Si-M-O (M = Ni or Sc; M/Si = 0.5) membranes synthesized on a mesoporous anodic alumina capillary (MAAC) tube using chemical processing techniques were characterized by transmission electron microscopy. The chemical composition of the membranes was analyzed by energy-dispersive X-ray spectroscopy. An as-synthesized Si-Ni-O membrane was found to be formed as a thin amorphous layer with a thickness of 400 nm on the MAAC tube, and in the mesopore channels of the MAAC tube, amorphous Si-Ni-O was also formed. The EDS analysis indicated a compositional gradient of Ni/Si for the Si-Ni-O membrane. The ratio of Ni/Si increased the further in depth from the surface of the membrane. After heat treatment in a hydrogen flow, a multilayered structure was formed, and nickel and nickel oxides were precipitated in the membrane. In the Si-Sc-O membrane, neither precipitation nor compositional gradient of Sc/Si were observed. The compositional change and precipitating behaviors of the Si-Ni-O and Si-Sc-O membrane are discussed in terms of the stability of the silica-dopant metal network and the hydrogen permselectivity of the membranes. (c) 2006 Springer Science + Business Media, Inc.