Transport characteristics of Nafion membranes, that have been published earlier, are re-evaluated. It is found that the specific conductivity of the membranes is not only determined by the mobility of the ions, but largely also by the interaction of ions with water and with microscopic membrane channel structures. Similarly, the water transference coefficient, defined as the number of moles of water transported per Faraday through the membrane, is governed by two effects : an electrostatic effect between ion and water dipoles, and an effect due to the size of the cation. Contributions to electro-osmotic water transfer are water of hydration to cations and hydrodynamically pushed water molecules. The size of the ion compared to the channel diameter, has a major impact on the electric conductivity, but also on water transport. It is shown that hydrophilic cations can promote an enlarged hydrophilic domain in the membrane, that is accompanied by a lower membrane resistance. Criteria for designing high performance ion conducting membranes are given based on this basis.