The polyimide-based X-ray windows are evaluated with respect to oxygen-induced erosion with an oxygen plasma asher. The use of the atomic oxygen beam was unsuccessful because of particle contamination problems. Various window types were compared with leak tests and microscopic analyses, which were correlated to the structural and fabricational information. The leak test was found to be a very sensitive method in through-the-membrane pinhole detection. The classic membrane structure was found inadequate at withstanding the oxygen erosion exposures. The multiple layer structure with spacer-polyimides between the multiple coatings was found to give excellent protection by restricting the erosion crater depths.