The emf method has been compared with the conductometric one in determining the apparent electrolyte permeability of membrane (P-s((v))). It has been found that this method is limited to the membranes of low permeability (P-s((v))/l(m) of the order 10(-7) m/s). In order to obtain the real permeability coefficient (P-s), the conductometric method of determining osmotic flow has been proposed. The influence of concentration polarisation on P-s has been estimated. The extended expression for the polarisation correction factor has been derived. The thickness of the polarisation layer has been calculated from the limiting current density, determined using the Nafion 117 membrane.