AbstractUsing the formalism of irreversible thermodynamics, general conditions are derived for osmotic salt and solvent permeation. It is found that in many types of low charged membranes abnormal osmosis may occur; for the more selective ion exchangers, however, solvent flow is usually directed towards the concentrated phase, whereas electrolyte flux may be parallel or opposed to the activity gradient, regardless of the nature of the membrane. Application of the theoretical results in a particular system requires membrane composition data and leads to a set of “friction coefficients” rik. Determination of all rik values even in a simple system like sodium chloride, water and membrane requires an extensive set of experimental data. Though the complete set has not yet been obtained, rik values could be estimated from data on osmosis, transference, conductivity and diffusion. The friction between co‐ion and membrane appears to have a predominant effect on dialysis efficiency.