A novel Cs+ion source combining the advantages of porous metal ionizers with those of zeolite emitters has been developed. Cesium ions are chemically stored in a cesium‐mordenite solid electrolyte (Cs‐M SE) pellet whose emitting surface is coated with a porous tungsten thin film. Cesium supply to the emitting surface is controlled by a voltage applied across the solid electrolyte pellet. Cs+ion emission takes place on the surface of the porous tungsten electrode by surface ionization. The ionic conductivity of Cs+ion in the Cs‐M SE is of order of 10−5&OHgr; cm−1at 1000 °C. The interface properties between electrolyte and electrodes play a major role in the cesium ion source. The cathode electrode interface (emitting electrode) determines the stability of the supply current density to the emitting surface. The lifetime of the source is found to depend on the anode interface. In a steady‐state operation, an ion‐emission current density of the order of 20 mA/cm2has been extracted for 30 h at a total ion‐emission current of 100 &mgr;A at 1000 °C. This corresponds to 10 C of extracted charge.