Sorption kinetics and sorption isotherms have been determined for dimethyl methylphosphonate, DMMP, in a perfluoroionomer, Nation(TM), in the acid form and as calcium and ferric salts. Nafion is a phase segregated polymer with a fluorocarbon main chain and sulfonic acid terminated fluoroether side chain comprising 33 wt% of the polymer. DMMP swells only the fluoroether region. Under immersion conditions, equilibrium solubilities for all three forms are essentially equal, averaging about 130 wt%. Kinetics are non-Fickian, dominated by relaxation effects, with calcium much slower than the acid form of Nation, and ferric slower still. Analysis with the Joshi-Astarita model of relaxation coupled diffusion indicates that the effect of cation modification is a more than 200-fold decrease in both diffusion coefficient and relaxation rate. Vapor sorption kinetics is also non-Fickian and dominated by relaxation, frequently involving a series of steps or several maxima at high vapor activity. Additionally, kinetics and, to a lesser extent equilibrium solubilities, are affected by sample history and conditions of sample exposure. The sorption isotherm, for acid Nation exhibits a rapid concentration increase at vapor activities below 0.2, a low slope at intermediate activities and a rapid increase above activity 0.8. Isotherms for salt modifications are similar but with even lower slope at intermediate activities. Effective diffusion coefficients for acid Nafion, calculated from boundary layer corrected steady state permeabilities combined with the sorption isotherm, exhibit a pronounced rise at a volume fraction of 0.6. A transition in NMR solvent self-diffusion coefficients attributed to the emergence of a new phase segregated structure is reported at the same concentration. When effective diffusion coefficients are converted to solvent self-diffusion coefficients, the values are 30-fold lower than NMR values at low concentrations, taken as a measure of tortuosity, but closely approach NMR values above the transition. The prominence of relaxation controlled sorption kinetics and high degree of tortuosity below the transition suggest that Nation is a disordered structure of small interspersed fluoroether and fluorocarbon regions, whereas the occurrence of discrete relaxation events, mainly at high activities, is probably associated with larger scale reorganization of the partially coalesced phases above the transition. (C) 2004 Published by Elsevier Ltd.