The limits for the sensitivity of spectroscopic ellipsometry (SE) to the presence of near-surface gradations of the oxygen content in YBa2Cu3O7-delta (YBCO) thin films have been investigated. Using an ellipsometric data modelling program, SE spectra were generated for films with fixed geometries and varying concentration gradients of oxygen. By comparing these spectra, the sensitivity limits of SE to small changes in the diffused depth and/or the surface concentration of oxygen (c(s)) were determined. For c-axis oriented films, it was found that the oxygen content can, in principle, be determined to within one unit cell of YBCO, even when the profile is graded. Also, the minimum detectable change in the surface oxygen concentration (c(s)) was calculated to be 2-3%, depending on the starting c(s). Similar calculations for YBCO layers buried under a 4 nm layer of metallic SrRuO3 (i.e. in a superconductor normal metal junction) showed small changes in these sensitivities. Calculations for a-axis oriented films showed that changes as small as 10 Angstrom in the diffused depth were detectable for both buried and surface layers. The minimum detectable change in c(s) for a-axis films was found to be 2-8%.