The sensitivity of ellipsometry to bulk surfaces and thin films in the extreme ultraviolet region is studied theoretically in reference to the complex refractive indices of the materials and the extinction ratio of a multilayer polarizer. At a photon energy of 97 eV, which is 12.8 nm in wavelength, sensitivities in terms of the minimum detectable differences to a refractive index of 0.027 and to an extinction coefficient of 0.048 can be expected for bulk surfaces. For a thin film sample, the sensitivities are 10(-3) nm for the thickness, 10(-4) for the refractive index and 10(-5) for the extinction coefficient. Ellipsometry in this wavelength region proves to be useful for the study of the near-surface region of the sample within a penetration depth of approximately 150 nm with good material and optical structure sensitivity.