Abstract The chemical and microstructural features of poly(styrenesulfonic acid) (PSSA)-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) polymer electrolyte membranes (ETFE-PEMs) through preparation procedures and grafting degree (GD) are investigated by solid-state 13C cross polarization/magic angle spinning solid-state nuclear magnetic resonance (13C CP/MAS NMR) spectra, atomic force microscope (AFM), X-ray diffraction (XRD), and Doppler broadening (DB) of positron-annihilation radiation measurement. Especially, sequential order in the structural changes as a response to GD increase is discussed in detail by using the two-dimensional correlation spectroscopy (2D-CS) analyses. The grafting process at –CH2 sites occurs earlier than that at –CF2 ones. The chemical degradation of ETFE-PEMs (even at a low GD of 8.8 %) observed previously...