Ru-capped Mo/Si multilayer mirrors were irradiated by EUV in a vacuum atmosphere with ethanol or decane gas, and their reflectivity changes by contamination were investigated by changing the amount of introduced gas. The reflectivity hardly decreased by EUV irradiation in the ethanol-introduced atmosphere. On the other hand, the reflectivity decreased by about 5% in the decane-introduced atmosphere at a decane pressure of P-Decane = 1.3 x 10 (4) Pa, an EUV power of about 200 mW/mm(2), and an EUV dose of 150 J/mm(2). EUV irradiation to the Ru-capped multilayer mirrors was also performed in the presence of water vapor and decane. The surface oxidation by EUV irradiation with a water vapor pressure of P-H2O - 1.3 x 10 (-5) Pa was controlled by the introduction of decane at a pressure of P-Decane = 7.0 x 10 (7) to 1.3 x 10 (6) Pa. (C) 2009 Published by Elsevier B. V.