Using a hydrogen pump cell, water flux, current density, and membrane resistance across a Nafion 115 membrane were measured as functions of temperature, water activity, and applied potential. Water flux increased with increase in water activity difference across the membrane. When a potential was applied, (i) the current increased with increase in applied potential up to an applied potential around 0.3V; (ii) the current did not increase with further increases in applied potential for applied potentials between 0.3V to 1V. The water flux continually increased with increase in applied potential, even when the current did not increase for applied potentials > 0.3V. Electro-osmotic drag coefficients were evaluated as a function of applied potential, water activity and temperature. The electro-osmotic drag coefficient (number of water molecules transported with the protonic current) was < 0.15 at low applied potentials and increased to around 0.35 at applied potentials of 1V. The results indicate that the conduction mechanism for protons in Nafion changes from proton hopping to electric field assisted motion of hydrated proton aggregates as the applied potential was increased.