The thermodynamics of alkali and alkaline earth metal ion exchange on a layered zirconium sulphophosphonate having the general composition Zr(O3PC6H4SO3H)×(HPO4)2−×yH2O have been investigated. Enthalpy and entropy changes accompanying the M2+- H+exchange (M = Na+, Cs+, Mg2+and Ba2+)were determined by the temperature variation method. For the monovalent ions, Na+and Cs+, the enthalpy terms favor exchange whereas the entropy terms are unfavorable. In contrast, for the divalent ions, Mg2+and Ba2+, the exchange is due to highly favorable entropy terms.