Ion exchange and selectivity of ammonium mordenite have been investigated for solutions of hexaquonickel (n) chloride and hexamminenickel (it) chloride in aqueous triethanolamine. Exhaustive exchanges of mordenite with the two salt solutions resulted in final exchange levels of 7% and 54% respectively. The differences between the exchange isotherms, and the unusual isotherm obtained with hexaquonickel, are explained in terms of a ternary ion exchange process involving ammonium, triethanolammonium and complexed nickel ions, in which volume-steric and ion-sieve effects influence the maximum nickel uptakes.