In order to prepare metal-loaded catalyst membranes for CO oxidation, gamma-Al2O3 layers were formed on porous alpha-Al2O3 support tubes (1.7 mm i.d., 2.1 mm o.d.) using a sol-gel technique. The layers were then impregnated with Pt, Ru, Ni, Cc and Rh, using aqueous solutions of H2PtCl6, RUCl3, Ni (NO3)(2), CO(NO3)(2) and RhCl3, respectively, and calcined in air at 573 K. A gaseous mixture of H-2, CO and O-2 was permeated through these membranes, and their activities were determined with respect to CO oxidation at 423-523 K. The CO oxidation activity was the highest for the Rh/-gamma-Al2O3 membrane. Based on the report that the CO oxidation activity was much reduced for higher concentrations of CO than the threshold value, a membrane was developed by forming a microporous silica layer on the Rh/gamma-Al2O3 membrane. By the selective permeation with no feed of O-2, the CO concentration of 50,000 ppm on the feed side could be decreased to 610-2700 ppm. When O-2 was added to the feed, the CO concentration was further decreased. Thus, the composite membrane, which was developed in the present study, can play dual roles: H-2-selective permeation with a high H-2/CO separation factor, and catalytic oxidation of CO which leaked through the microporous silica layer.