The adsorption of CO at high pressures (up to 120 Torr) on a hydrocarbon-precovered Mo(100) surface is studied by reflection-absorption infrared spectroscopy (RAIRS). It is shown that CO adsorbs reversibly on the Mo(100) surface even in the presence of a thick (similar to C-8) carbonaceous layer and forms a saturation coverage of about 20% of that on clean Mo(100). Although CO is used as a probe molecule, this result confirms that, under high pressure, the reactants still can adsorb on the catalyst in spite of the presence of a thick carbonaceous layer formed under the reaction conditions.