In order to apply magnesium-selective microelectrodes to intra-and extracellular measurements, a careful optimisation of the membrane composition, and of the electrode geometry is necessary. The reasons and strategies are discussed in the first section. When using a lipophilic 2-nitrophenylether derivative, ETH 8045, as a plasticiser, and 155 mol% anionic sites as additives relative to the ligands ETH 3832 and ETH 5506, calcium ions are discriminated by a factor of 32 (ETH 3832) and 100 (ETH 5506), and sodium ions are discriminated by factors > 10(4) when measured in a continuous flow system at 37 degrees C (SSM). Using "Hofmeister"-microelectrodes as detectors eg in capillary electrophoresis is also attractive due to the high discrimination of ions which exhibit a high free energy of hydration such as magnesium or lithium ions or TRIS (tris-(hydroxymethyl)-aminomethane) recommended as buffer and baseline components. The selectivity pattern of typical "Hofmeister"-electrodes favourably used in order to achieve a high transient signal for lipophilic organic cations such as histamine relative to the low baseline potential raised by magnesium and other hydrophilic cations. An appropriate geometry of the capillary end was shown to decrease the local field strength and, therefore, the noise of the signal. This is another example where a synergetic effect of an appropriate geometry and of an intelligent sensor design improves the performance of the analytical method relevantly. A mathematical description of the differential peak response is given and an electropherogram of an application in red wine is shown.