In the adsorption isotherm approach, ion exchange should be considered as a competitive adsorption of two sorbates. This would lead to adsorption isotherms equivalent to the law of mass action. The simple Langmuir equation, representing a single-sorbate system, should not be used to represent ion exchange even at low occupancy values of an ideal reaction. The application of this equation generally lends to erroneous values of selectivity constants. In case of heterovalent exchange, it also leads to erroneous capacity values. These facts have been revealed by the isotherm of Boyd et al. [G.E. Boyd, J. Schubert, A.W Adamson. J. Am. Chem. Sec. 69 (1947) 2816] for homovalent exchange, and by the approximate isotherm of Misak for heterovalent exchange. Cases of constancy of the equilibrium quotient of equivalent ion exchange are numerous. Adsorption isotherms can also be applied to the more frequent cases where such constancy is only observed over a certain composition range of the solid. Binding constants represented by the simple Langmuir equation are actually not constants, being dependent on solution concentration. Except under certain particular conditions, such constants are generally meaningless. (C) 2000 Elsevier Science B.V. All rights reserved.