Ion-exchange isotherms have been measured at 25°C for the high framework charge chabazite-type zeolite, K-G1, and involving the cation pairs: K⁺⇌ Na⁺, Na⁺⇌ Li⁺, Na⁺⇌ Cs⁺, Na⁺⇌ Ca²⁺, Na⁺⇌ Sr²⁺, and Na⁺⇌ Ba²⁺. The results have been compared with those for an isostructural natural chabazite of lower charge density. The values of the thermodynamic equilibrium constants show that the larger ion is preferred by the zeolite of lower charge density. In terms of a solid state reaction involving exchange of ions between both zeolites, the standard free energy of exchange is negative if the larger ion goes from G1 to chabazite. All these results can be explained in terms of dielectric theory if the local dielectric constant in solution is greater than that in either zeolite, and if this constant is greater in a given cationic form of the zeolite of high charge density than in the corresponding form of the zeolite of lower charge density. The correctness of these assumptions for the zeolites was supported by measurements of the refractive indices. The exchanges involving caesium are incomplete in both G1 and the natural chabazite. The different limiting values for these two exchangers have been discussed in terms of steric, electrostatic and hydration effects. The pattern of exchange behaviour observed is consistent with that reported previously for the isostructural zeolites X and Y.