Potash has been extensively used as fertilizers and industrial materials. To utilize the potassium resources in the seawater effectively, a potassium ionic sieve membrane was synthesized on alpha-Al2O3 ceramic tube by the hydrothermal synthesis method. Then it was characterized by SEM photos, which showed that it was closely combined with the support and there were no obvious intracrystalline pores or fissures. Afterwards, the separation selectivity of the potassium ionic sieve membranes to K+, Na+, Ca2+ and Mg2+ was determined to evaluate the potential of its application on extracting potash from seawater. The results showed that the separation factors of K+/Na+, K+/Mg2+ and K+/Ca2+ were equal to 93.5, 161.4, 1185.7 respectively, which indicated that the potassium ionic sieve membranes had much higher selectivity to K+. Further studies on the continual ionic exchange experiments using the potassium ionic sieve membrane proved that the flux of K+ across the potassium ionic sieve membrane increased with the increasing of flow rate, and higher temperature was benefit to the separation of K+ in K+/Mg2+ mixed solutions. The continual ionic exchange process can proceed on the potassium ionic sieve membrane spontaneously without any external power. Furthermore, a mechanism based on the channel diameter of the potassium ionic sieve membrane was proposed to explain its higher selectivity to K+. (C) 2012 Elsevier B.V. All rights reserved.