A moderately charged cation exchange phenolsulfonic acid membrane was investigated electrochemically as a separator between NaCl solutions in concentration cells with Ag/AgCl electrodes. Initial time experiments using small concentration differences as previously described (J. Phys. Chem. 1994, 98, 9013) indicated very little asymmetry in the membrane specimen used. From the initial time values of emf the transport number is calculated asa function of salt concentration, assuming the influence of H+ ions on the emf to be negligible. Stationary state emf values were also measured between concentrations of NaCl equal to 0.01 and 0.08 mol L(-1). A considerable influence of the stirring rate was observed. In well-stirred experiments the stationary state value is 79.5 +/- 1.0 mV compared to the value 78.3 +/- 1.0 mV obtained by performing the transport number integration using initial-time emf datal This excludes also the presence of other significant macroheterogeneities (with surface symmetry).