Abstract
1. The membrane potentials measured at the BaSO~4~โcellophane membrane point to selectivity of its permeability, supporting the conclusions drawn from previously reported direct permeation experiments.
2. In the presence of permeating ions other than H^+^ and OH^โ^, the membrane potential at the BaSO~4~ membrane is not exclusively dependant on the pH's of the adjacent solutions. Other ions, e.g. K^+^ and Cl^โ^, may even take over the predominating role in the development of the membrane potential โ in nonโquilibrium states.
3. In nonโequilibrium systems with other permeating ions besides H^+^ and OH^โ^, spontaneous ion redistribution takes place and the system changes towards a Donnan equilibrium. The rate at which these changes take place greatly depends on the composition of the two adjacent solutions. The changes are often very slow.
4. The eventually reached equilibrium state is a (double) Donnan equilibrium. The fact that Donnan equilibria could be established under favorable conditions, is a further proof for the selective permeability of the BaSO~4~ membrane.
5. An electrochemical hypothesis is given as a possible explanation of the observations on BaSO~4~ membranes so far described.