Abstract: An ion-sensitive measuring electrode device with an ion-sensitive element based on ion-conducting crystalline ceramic material is provided. The electrode device shows a selectivity to the Na.sup.+ ion versus the H.sup.+ ion corresponding to the selectivity of the known sodium-sensitive glass-based solid membrane electrodes.Ion-sensitive measuring electrode devices showing good selectivity properties are provided on the basis of ion-conducting crystalline material, the crystal structure of which comprises a three-dimensionally extending interstitial space containing positions for the ion, especially material in which the interstitial space has bottlenecks which just permits passage of the ion.A method for preparing polycrystalline ceramic material based on oxides of zirconium, phosphor and silicon is also provided.