The preparation and characterization of nanoscale metal sulfide particles in perfluorinated ionomer membranes, including Nafion and those based on the sulfonimide ionomer and the bis(sulfonyl)methane ionomer, are reported. The results suggest that these membrane films all contain hydrophilic structural cavities and that the nanoscopic structures of the sulfonimide ionomer and bis(sulfonyl)methane ionomer membranes are quite similar to that of the commercial Nafion film. Properties of the nanoscale particles in the ionomer membranes are presented, and their implications for the understanding of membrane nanoscopic structural details are discussed.