A deduction of a relation between transport coefficients of ion-exchange membranes is considered by comparison the Kedem-Katchalsky and Onsager forms of transport equations in the framework of irreversible thermodynamics. This relation is analysed by using the experimental data of Narebska et al., Berezina ct al., and the results of other authors. Transport equations generalising the Nemst-Planck equation with the coefficients determined directly from the practical transport characteristics are obtained either taking into account or disregarding the above mentioned relation. The Nernst-Einstein relation and its generalizations are discussed.