Calorimetric and NMR titration experiments have been done to measure the binding constant between p-sulfonatocalix[4]arene and a quaternary ammonium ion. Our results show that the binding constants depend both on the calixarene concentration and on the presence of added Na+. These results have been interpreted by considering the ion-exchange equilibrium between sulfonatocalixarene counterions and the added organic cation. Our results show that it is necessary to extrapolate the binding constants to zero calixarene concentration and zero added salts in order to get the true equilibrium constant.