The results of a detailed study of the ternary exchange systems Ni/Cu/K and Ni/Cu/Na in zeolite Y are presented and discussed. The Ni/Cu/K-Y ternary exchange isotherm (T = 298 K) was constructed from eighty-five pairs of experimental points which provide a comprehensive coverage of the possible range of solution compositions. Selectivity and thermodynamic data for the Ni/Na, Ni/K, Cu/Na and Cu/K binary systems are also provided. Isotherm reversibility, incomplete exchange, hydrolytic cation loss and metal hydroxide precipitation are addressed. The selectivity trends observed in the ternary exchanges are described quantitatively in terms of ternary and pseudo-binary separation factors. Zeolite Y exhibits a much stronger preference for copper over nickel. Copper is also favoured over the parent alkali metal cations in the large cages regardless of the nickel content whereas nickel is only preferred over sodium or potassium at very low levels of copper exchange. The nature of the out-going cation is shown to exhibit a considerable influence on the progress of exchange in both binary and ternary systems: the large cage sodium ions offer a lower resistance than potassium to exchange with the external transition metal ions while the reverse is the case for the small cage alkali metal ions.