Adsorption of CH4 and coadsorption of CH4 and CO on differently treated magnesium oxide have been studied by FT-IR spectroscopy at 173-273 K. Five IR bands at 3008, 3000, 2900, 2890, and similar to 1306 cm(-1) were observed when well-outgassed MgO was exposed to CH4 at 173 K. The bands at 3008 and 3000 cm(-1) are assigned to a degenerate Stretch vibration of CH4, and those at 2900 and 2890 cm(-1) originate from an infrared-forbidden mode at 2917 cm(-1) of free CH4. These bands became noticeably stronger with the outgassing temperature of Mgd but could be sufficiently reduced as MgO was pretreated in air or under water vapor. Adsorbed CO on MgO gives two IR bands at 2161 and 2155 cm(-1) which exhibit a parallel variation with those bands of adsorbed CH4 with the different pretreatments of MgO. Coadsorption of CH4 and CO indicated that adsorbed CO can deplete part of the adsorbed methane, particularly that with IR bands at 2900 and 2890 cm(-1). It is proposed that the adsorbed methane can be substituted by adsorbed CO coordinated;l to a Lewis acid-base pair site, MgLC2+OLC2-, and the methane unaffected by coadsorbed CO interacts mainly with surface oxygen anions alone. The observed IR bands can be attributed to two types of adsorbed methane on MgO : one with IR bands at 3008 and 1305 cm(-1) due to weakly adsorbed methane on surface oxygen anions and another with bands at 3000, 2900, 2890, 1305 cm(-1) due to methane strongly interacting with the Lewis acid-base pair sites of MgO. A possible mechanism of methane interaction with surface of MgO is discussed.