A study of proton conductivity in a commercial sample of Nafion(R) 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidify conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox-initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four-point-probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a "liquid-like" mechanism of proton conductivity whereby protons are transported as hydrated hydronium ions through water-filled pores and channels in the ionomer.