To protect the surface oxidation of Mo/Si multilayer films by extreme ultraviolet (EUV) irradiation under a vacuum atmosphere with residual water, two experiments were carried out. One consisted of examining the oxidation protection effect for isopropyl alcohol (IPA) and n-decane gases. The reflectivity change of the Ru-capped multilayer film by EUV irradiation was investigated under a vacuum atmosphere with residual water vapor at a pressure of 1.3x10(-5) Pa, and, in addition, each hydrocarbon (HC) gas was introduced by changing its pressure. A protective effect against oxidation was observed in both gases when introduced at a pressure in the order of 10(-6) Pa. For IPA, no remarkable decrease in the reflectivity was observed even when the introductory pressure was raised to the order of 10(-4) Pa. However, for n-decane, the reflectivity decreased remarkably when pressure in the order of 10(-5) Pa was introduced. The other experiment consisted of examining the reduction effect of the oxidized surface by EUV irradiation when introducing HC gas. Ru- and Si-capped multilayers were once oxidized by EUV irradiation under a water vapor atmosphere. However, for the Ru-capped multilayer, the reflectivity was recovered when EUV was irradiated in the presence of ethanol gas at a pressure of 3.8x10(-5) Pa. The oxide layer of the Ru cap was reduced at this time.