A membrane electrode has been developed which is responsive to nitrate and other univalent anions but non-responsive to sulphate and other multivalent anions and to cations other than hydrogen. The membrane is prepared by polymerizing a mixture of phenol, formaldehyde, ammonia and nickel nitrate directly to a film for mounting in the usual cell-with-membrane assembly. Response to hydrogen ion is 0.30 mV per pH unit, to nitrate 0.60 mV per pNO(3) (activity) unit. Based on chemical composition and behaviour, physical properties, and electrical response, a structure and mechanism for exchange with hydrogen ion and nitrate ion and for internal electrical conduction has been postulated. Applications have been made to the determination of nitrate and to the detection of end-points.