Comprehension of gas phase transport processes and species distribution in operating polymer electrolyte fuel cells (PEFC) is an important task for understanding and optimizing the related coupled processes. Few efforts for in-situ gas analysis in fuel cells are described in the literature. and previous work has focused mainly on integral species concentration or investigation in small laboratory scale cells under comparably dry conditions only. In this work. a methodology for flexible local in-situ gas analysis in polymer electrolyte fuel cells of technical size is presented. This includes conceptual description of measurement modes during fuel cell operation in particular for local membrane and gas diffusion layer characterization Beside characterization of the analysis system, the potential of the method is demonstrated by accurate local concentration measurement of gas species including water vapor up to saturated conditions Accurate measurement of local membrane tracer gas permeation under open circuit and load conditions emphazises the flexibility of the developed methodology In particular, the ability to determine local membrane permeation characteristics is of great value for membrane degradation investigation (C) 2009 Elsevier B.V All rights reserved