Na-β-gallate is an alkali fast ion conductor which is isostructural with β-Al2O3. Previous work had shown that the conductivity of crystals having mixed (Na, K) compositions is greatly reduced over that of the pure sodium and potassium compounds. The anomaly is similar to the "mixed alkali effect" observed in glasses and amorphous materials. Crystallographic investigation of the pure compounds and a crystal in the vicinity of the conductivity minimum has revealed that the drop in conductivity is accompanied by site preferences of Na+ and K+. The conduction plane is characterized by three distinct sites for the alkali ions, a normal Beevers-Ross lattice site and two interstitial sites. K+ is present on the normal lattice sites only. Na+ is exclusively present on the interstitial sites. © 1981.