In extreme ultraviolet (EUV) lithography, a surface-imaging process using an organosilicon resist is essential. Since the binding energies of Si core-level electrons coincide with the photon energy of the light source of EUV-lithography, we have analyzed the effect of their excitation on the decomposition of an organosilicon resist. We have also investigated a design policy for optical systems in terms of selecting the multilayer mirror materials to use organosilicon resists in EUV lithography. A photon-stimulated ion desorption (PSID) analysis of poly(cyclohexylmethylsilane) (PCHMS) that was exposed to monochromatic synchrotron radiation was carried out. We found that decomposition of the Si–Si backbone of PCHMS is enhanced by EUV exposure to wavelengths that excite the Si 2p and 2s electrons. In addition, the yield of CH3+ desorbed from the side-chain methyl of PCHMS is increased at a photon energy of 108 eV (a 11.5-nm wavelength), which presumably excites Si 2p electrons into the Si–C σ* state. Calculated reflectivities of a multilayer mirror suggest that the reflectivity of a multilayer mirror which does not contain Si, such as Mo/Be, realizes excellent reflectivity at the wavelength that excites the Si 2p electron.