A valinomycin-based membrane phase for microelectrodes with relatively low electrical membrane resistances is described. Microelectrodes with tip diameters of about 1 micron exhibit resistances of about 10(10) omega. Extremely high K+ selectivities are obtained, e.g. a rejection of Na+ by a factor of 5000 and of acetylcholine by a factor of 3400. At a constant background of 140 and 500 mM Na+, the detection limit of the K+ sensor is at 1.6 X 10(-5) and at 2.5 X 10(-5) M K+, respectively.