An intracellular ion-selective microelectrode based on the neutral carrier ETH5214 with chlorinated paraffin (60% Cl) and o- nitrophenyloctylether (o-NPOE) as solvents is presented. Compared with the sensor based on the same ionophore with solvent o-NPOE, the potentiometric selectivity factor for Mg^2^+ with respect to K^+ has been improved by 0.9 order of magnitude. The microelectrode shows a near-Nernstian response in pure MgCl~2 solutions and in the MgCl~2 solutions at a typical intracellular ion background, with detection limits of 4×10^-^7 mol.dm^-^3 and 2×10^-^5 mol.dm^-^3, respectively. The H^+ does not interfere with the emf response of the electrode in pH 3.5-9.5. The lifetime of the microelectrode cell assembly is longer than 5 days, and the response time (t~9~5)≤3s.

Key words: MICROELECTRODE, ION SPECIFIC ELECTRODE, MAGNESIUM ION, ION SELECTIVE ELECTRODE, NITROBENZENE, PARAFFIN WAXES, RESPONSE TIME