SummaryMost constituents of the adsorbing complex of the soil are cationic exchangers with variable exchange capacity: it is possible to study graphically exchanges involving the protons and two other cations. The representation developed here uses and generalizes the principle of the bicationic exchange isotherms in a square diagram. This helps to explain some experimental phenomena and enables the final equilibria of the exchange reactions to be forecast. The model seems useful for application in agronomy and physiology.