Zeolite Na-P has been synthesised and studied as an exchange medium at 25 °C and for 0·100N solutions for the ion pairs Na ⇌ Li, Na ⇌ K, Na ⇌ Rb, Na ⇌ Cs, 2Na ⇌ Sr, and 2Na ⇌ Ba. All exchange isotherms were reversible. Among the selectivities observed two were of particular chemical interest: the preference of the crystals for Na over Li and for Ba over Na. These are sufficient, with other observed selectivities, to suggest the use of this easily synthesised zeolite to remove alkali-metal and alkaline earth-metal ions from Li salt solutions, and Ba from sodium salt solutions, in each case under neutral or alkaline conditions. The crystals also show good selectivity for Sr.Both water removal and ion exchange are associated with reversible structural changes in the rather flexible gismondite-type framework of Na-P. These changes appear to exercise a considerable influence upon isotherm contours, selectivities, thermodynamic equilibrium constants, and activity coefficients of exchange ions within the zeolite.