When solute particles are distributed inhomogeneously in a solution due to the action of an external force, the pressure difference across a semipermeable membrane is determined by the concentration of the impermeant particles in the immediate vicinity of the membrane. In line with this conclusion, it is shown that in a Donnan system containing a porous membrane, the concentration of the impermeant polyelectrolyte at the membrane interface is equal to the difference between the osmolar concentrations of the solutions present on the two sides of the membrane. That is, the impermeant species are accumulated at the membrane interface in proportion to their density in the solution. The well-known effect of the concentration of the permeant ions on the equilibrium pressure difference in Donnan systems is related to the influence of the ionic strength on the extent of accumulation of the impermeant electrolyte at the membrane interface. Predictions derived from these theoretical considerations were verified experimentally.