A liquid membrane electrode has been developed with Nernstian response and a high selectivity for carbonate ion. The electrode responds rapidly to carbonate in the 10(-2)M to 10(-6)M range with a selectivity for carbonate over chloride, sulfate, and phosphate of at least 10(3) and negligible bicarbonate response. The electrode properties appear to be suited to chemical, biological, chemical, and oceanographic measurements.