Self-diffusion constants for water, ethanol and decafluoropentane were measured in the perfluorosulfonate ionomer Nafion (R) using pulse field gradient proton NMR. Measurements were made as a function of concentration, temperature and diffusion time. For water, diffusion constants ranged from 10(-4) to 10(-7) cm(2)/s. A reasonable interpretation could be developed using free volume theory if the concentration was scaled by considering only the ionomeric pendant group as the accessible domain. The temperature dependence could be summarized in terms of the WLF equation. For ethanol, the self-diffusion constants are comparable in magnitude to those of water but the concentration dependence is stronger and the temperature dependence weaker. Free volume theory provides a poor framework for summarizing the concentration and temperature dependence of the diffusion of ethanol in Nafion. in an accompanying report, fluorine-19 spin diffusion and fluorine-19 line shape data indicate larger morphological changes in Nafion upon addition of ethanol and greater plasticization of the pendant group domain. The larger morphological changes are thought to contribute to the concentration dependence of diffusion making free volume theory inapplicable. The decafluoropentane diffusion constants appear to depend on the time scale over which the diffusion is observed. This is not true for the other two penetrants, which are considered to be primarily located in domains comprised of the sulfonate groups and side chains. The decafluropentane is likely to be located in amorphous domains composed of the CF2 groups. The dependence of the self-diffusion constant of the decafluoropentane on diffusion time indicates that the domain or phase supporting transport is poorly interconnected. At long diffusion times, the product of the self-diffusion constant and the diffusion time becomes constant which is associated with restricted diffusion.