The effects of germanium ion incorporation into tin oxide on the selectivity for the selective oxidation of methane in the presence of NO over Sn1-xGexO2 (x = 0-0.5) were examined with the use of a conventional catalytic reaction and theoretical calculations. The reactivity of the tin oxide for methane oxidation was decreased by the incorporation of germanium. The product selectivity was measured at 10% methane conversion. The product was CO2, CO, and a small amount of C-2 compounds in the absence of germanium, but we obtained Cl-oxygenates over every germanium ion-incorporating catalyst. The main product of the Cl-oxygenates was formaldehyde. The selectivity of formaldehyde increased with the content of germanium (x) and had a maximum value at x = 0.2. We considered two reaction routes of HCHO formation: a heterogeneous reaction and a homogeneous one. Since a larger proportion of CO2 selectivity in comparison with CO selectivity was found in the presence of Sn0.9Ge0.1 O-2, we assumed that the heterogeneous reactions over the catalyst were dominant in a lower temperature region below ca. 860 K. We suggested that the C-1-oxygenates were produced on the Sn0.9Ge0.1O2 catalyst, not in the gas phase. We theoretically predicted that the substituted germanium ion Occupied the site for fourfold Sn2+ on the topmost layer by the DFT theoretical calculations. From the results of calculation, we assumed that the coverage of chemisorbed oxygen on the Ge/SnO2 (110) surface was expected to be smaller than that on the SnO2 (110) surface through the reactions, and we concluded that the smaller amount of chemisorbed oxygen through the reactions and the obstruction of oxygen migration over the Ge/SnO2 was favorable to an increase in the selectivity for C-1-oxygenates in the reaction. (c) 2005 Elsevier Inc. All rights reserved.