Perfluorosulfonic acid ionomers have been widely used as proton-conducting materials in polymer-electrolyte membrane fuel cells and their dispersions in polar solvents are used to prepare proton-conducting membranes as well as catalyst layers. While it has been known that the dispersed particles become monodisperse after heating to 230 degrees C or above, their size and shape have not been fully characterized yet. Cryogenic transmission electron microscopy images and small-angle X-ray scattering profiles of the dilute aqueous dispersions before and after heating to 240 degrees C have clearly shown that rod-like particles became significantly shorter cylinders without changing their radii. Particle length distributions were lognormal and average molar mass and dispersity after heating to 240 degrees C were consistent with the values from size exclusion chromatography. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 813-818