Ion exchange equilibria involving K cannot be described by a constant selectivity coefficient for all surface compositions. The main reason is that the soil exchanger contains various types of sites with different affinities for K ions. This paper makes use of a multi‐site model, the main assumption of which is that the exchange reaction is governed by single values of the Vanselow selectivity coefficient, K_v, for each type of site. The simplest model—i.e., the two‐site model—is tested. Three parameters must be adjusted: the proportion of high and low selective sites, and the K_v's for these two classes of sites. Simulated K‐Ca exchange isotherms are shown, with various combinations of the parameters. The model has been adjusted to experimental K‐Ca exchange data, using a non‐linear regression procedure. A fair agreement between the experimental and calculated curves is obtained. As a consequence, this simple model proves to be a powerful tool for a quantitative description of exchange data involving K ions.