We have demonstrated earlier the useful performance of our PSI radiation-grafted membranes in terms of the current-voltage characteristics of 30 cm2 active area fuel cells containing these membranes and their long-term testing over 6,000 h at 60 °C. We report here on testing of PSI radiation-grafted membranes in these fuel cells at 80 °C and in short stacks comprised of two or four 100 cm2 active area cells. The in-situ degradation of membranes has been investigated by characterizing membranes both before testing in fuel cells and post-mortem after testing in fuel cells. Characterization was accomplished by means of ion-exchange capacity and infrared and Raman spectroscopic measurements. In addition, a rapid screening method for our ex-situ testing of the oxidative stability of proton-conducting membranes was developed in this work. Comparison of the initial screening test results concerning the oxidative stability of some perfluorinated, partially-fluorinated, and non-fluorinated membranes compare well qualitatively with the relative stability of these same membranes during their long-term testing in fuel cells.