Concentrations of non-ionic species in aqueous media are measured as a function of potentiometric or polarographic response, employing a membrane electrode selective for the non-ionic species being measured. The electrode comprises a conductive substrate and a membrane formed as a layer on the conductive substrate and adapted for interfacing with the aqueous medium. The membrane is composed of a polymeric matrix having dispersed or dissolved therein an anion exchange material and a water-insoluble salt of the non-ionic species being measured. The electrode is particularly suitable for use as an implantable glucose sensor for continuous in vivo monitoring of glucose concentration in the blood or interstitial fluid.