1. The Teorell, Meyer-Sievers theory characterizes the electrochemical behavior of membranes by their selectivity constant "Ap" which is derived conventionally from concentration potential measurements at various concentration levels. The selectivity constant may, however, be derived also from entirely independent, different experimental data, namely base exchange studies. The constants arrived at in this second way are designated as "Ab." The selectivity constants derived by these two methods must be in reasonable, at least semiquantitative agreement if the basic assumptions of the theory are correct.
2. The selectivity constants Ap and Ab were determined for eleven different sets of membranes of different electrochemical activity and of different (8.2 to 80 volume per cent) water content.
3. The potentiometric selectivity constants Ap are in most cases several orders of magnitude greater than the corresponding Ab values. With membranes of great porosity and high electrochemical activity the Ab values approach at least in order of magnitude the Ap values.
4. It is concluded that the unexpectedly large discrepancy between the Ap and Ab values is due to some inherent weakness of the Teorell, Meyer-Sievers theory, most likely to its neglect of any structural factors.