The single-ion diffusion coefficients of H+ and MePEG(7)SO(3)(-) ions have been electrochemically measured in an anhydrous protonconducting electrolyte composed of MePEG(7)SO(3)H acid dissolved in our MePEG(7) polymer. H+ diffusion in this electrolyte is dependant on temperature. At low temperatures, H+ is the primary charge carrier (tau(H+) = 0.95 at 25 degrees C). However at high temperatures, the MePEG(7)SO(3)(-) anion diffusion coefficient is larger than DH+ yielding a smaller fraction of charge carried by H+ (tau(H+) = 0.29 at 85 degrees C). We conclude that our electrolyte is dominated by Grotthus conductivity at low to moderate temperatures, while the vehicle mechanism becomes dominant at high temperatures. (c) 2005 The Electrochemical Society.