Ionomer membranes are crucial components ofmany electrochemical devices. In this work, confocal Ramanmicroscopy is employed to characterize Nafion ionomers quantitatively in pristine status and after usage as a proton exchange membrane in a fuel cell. Confocal Raman microscopy allows non-destructive thickness and equivalent weight measurements of Nafion with a 95% confidence interval of +/- 13 g mol(-1) at an equivalent weight of 1000 g mol(-1), which is significantly more accurate than previously reported methods. Characterization can be performed at a spatial resolution better than 2 mu m, providing insights into local membrane degradation after fuel cell operation. Membrane thinning to less than 40% of the initial thickness of Nafion NR-211 occurs after a 100 h open circuit voltage hold, accompanied by an anisotropic increase of the equivalent weight from 1035 g mol(-1) to an average of 1200 g mol(-1). Most pronounced increases are found close to the anode. Further, the characterization of a Nafion XL membrane shows that its microporous reinforcement is represented as increased equivalent weight with local heterogeneities within the membrane. These results show that confocal Raman microscopy is a valuable tool to investigate ionomers that are used as ion exchange membranes in electrochemical devices. (c) The Author(s) 2019. Published by ECS.