Initial membrane potentials of two non-ionic polysulphonic (PS) membranes, used in hyperfiltration experiments, have been measured in an electrochemical cell filled with NaCl solution using Ag/AgCl electrodes. The concentration in one chamber (left chamber of the cell) was fixed to be 0.01 M. In the other chamber of the cell the concentration was varied between 10(-4) and 0.5 M. The two membranes were (1) a dense, almost uniform membrane, and (2) a porous, asymmetric membrane. It is possible from these initial time EMF measurements to evaluate the transport numbers of the sodium ion corresponding to each of the two surface layers of the membrane. From these transport numbers, an electrochemical characterization of the asymmetry in hyperfiltration membranes can be made. A comparison with the mean transport number obtained by stationary state EMF measurements is also performed. This transport number is always greater than the transport numbers obtained by the initial state method. The increased asymmetry of the membrane No. 2 is reflected in the initial time transport numbers (especially at low salt concentrations) but only to a minor extent in the stationary state mean transport numbers.