The　surface　state　of　Rh/MgO　catalysts　modified　with　Co,　Ni,　Fe,　or　CeO2　after　the　reduction　and　partial　oxidation　pretreatments　as　well　as　during　the　catalytic　partial　oxidation　of　methane　has　been　investigated　by　FTIR　of　adsorbed　CO.　The　results　of　CO　adsorption　on　the　reduced　catalysts　suggest　the　formation　of　Rh-M　alloy　on　Rh-M/MgO　(M　=　Co,　Ni,　Fe)　and　Rh　particles　partially　covered　with　reduced　ceria　on　Rh-CeO2/MgO.　The　strength　of　CO　adsorption　on　Rh/MgO　is　weakened　by　the　modification　with　Co,　Ni,　Fe,　or　CeO2.　Partial　oxidation　pretreatment　of　Rh/MgO　leads　to　a　significant　decrease　in　the　CO　adsorption　due　to　the　oxidation　of　Rh.　In　contrast,　on　partially　oxidized　Rh-M/MgO　(M　=　Co,　Ni,　Fe)　and　Rh-CeO2/MgO,　the　preferential　oxidation　of　the　surface　M　atoms　or　reduced　ceria　maintains　the　metallic　Rh　and　preserves　the　CO　adsorbed　on　the　surface　Rh　atoms.　The　CO　adsorption　during　the　reaction　of　catalytic　partial　oxidation　of　methane　on　Rh/MgO　and　Rh-Ni/MgO　is　similar　to　that　on　the　reduced　catalysts.　On　the　other　hand,　the　CO　adsorption　during　the　reaction　on　Rh-Co/MgO,　Rh-Fe/MgO,　and　Rh-CeO2/MgO　is　different　from　that　on　the　reduced　catalysts,　and　this　is　related　to　the　structural　change　of　these　catalysts　during　the　reaction.