Ion-exchange equilibria of alkali metal ions and protons of a manganese dioxide material have been studied thermodynamically. The material was synthesized by pyrolysis from a mixture of manganese carbonate and potassium tert-butoxide with content of 40% on a molar basis, at 530 degrees C. The chemical composition of the material is MnO2.09. 0.40H(2)O, and its crystal structure belongs to a hexagonal system with lattice constants a = 22.14 +/- 0.03 Angstrom and c = 4.900 +/- 0.002 Angstrom, which has not been reported in JCPDS. The ion-exchange titration curve showed that the material exhibited a property of weak acid, and the maximum uptakes were 3.67 meq Li/g, 2.59 meq Na/g, 2.54 meq K/g, 2.18 meq Rb/g, and 2.10 meq Cs/g. The plot of the corrected selectivity coefficient vs. fractional exchange (Kielland’s plot) was almost straight for Li+/H+, Rb+/H+, and Cs+/H+ exchanges, but curved for Na+/H+ and K+/H+ exchanges. The selectivity order at infinitesimal exchange is Li < Na < Cs much less than K < Rb at 30 similar to 60 degrees C. The pore structure in the material is expected to fit with K+ and Rb+. The negative Gibbs free energy change for K+/H+ and Rb+/H+ exchanges (Delta G(idealexch)(0) (K) = -22 kJ/mol and Delta G(idealexch)(0) (Rb) = -27 kJ/mol) showed that these exchange reactions were preferable in the present material. The entropy change was large for K+/H+ and Rb+/H+ exchanges (Delta S-hyp exch(0) (K) = 282 J/K . mol and Delta S-hyp exch(0) (Rb) = 458 J/K . mol), indicating high selectivities for K+ and Rb+ in the present material. The small Delta S-hyp exch(0) for Cs+/H+ exchange may express the size misfit of the ion with the opening of the exchange site.