Ion and pore fluid transport properties of an expanded form Nafion(R) membrane between two electrolyte solutions are studied by impedance measurements. It is shown that the rate of the pure fluid flow represents a kinetic factor that is responsible for a lower or even negative membrane impedance in the presence of alkali metal cations and protons. In spite of the generally faster ion transport in the expanded form, its impedance behavior differs slightly from that of an as-received form studied previously. This is likely to be due to a similar effect of the membrane composition and morphology on both the permeability and ion diffusion coefficient, the ratio of which influences the membrane impedance. By using the expanded H form Nafion as the reference system, the permeability of coefficients of the membrane are estimated and the following values of the membrane pore diameter are inferred for Li+, Na+, K+, Rb+, and Cs+ in an expanded (as-received) Nafion : 25 (14), 23 (19), 22 (-), 17 (-), and 14 (8) Angstrom, respectively.