The effect of chemical and mechanical membrane degradation on the gas separation is investigated in the early stage. The aim is to identify the trigger processes which lead to a nonuniform degradation. Chemical and mechanical degradation are investigated in accelerated stress tests at OCV, relative humidity cycling and a combination of both. Using a tracer gas concept, gas permeation in perfluorosulfonic acid membranes (Nafion) is analyzed locally and online in terms of diffusive and convective gas transport. Local phenomena, such as humidity fluctuations, cracks in the micro porous layer and carbon fibers, are found to initiate the degradation of the gas separation. Pinhole formation, induced by mechanical degradation processes, is crucial for the onset of a synergetic effect. Combined chemical and mechanical degradation accelerates pinhole growth, which is the major source of the increasing gas permeation in Nafion membranes with stabilized end-groups. The spatial nonuniformities of the relative humidity and humidity fluctuations are the main source of inhomogeneous membrane degradation. (C) 2012 Elsevier B.V. All rights reserved.