Abstract: The invention is directed to an electrochemical measuring cell for determining ammonia and its derivatives in a gaseous or liquid measuring sample. The measuring cell has at least one anode and at least one cathode which are disposed in an electrolyte chamber filled with a soluble electrolyte. The electrolyte chamber is closed off in the direction facing toward the measuring sample by a permeable membrane. The measuring cell provides a more selective ammonia sensor having the following advantages: a short response time, a linear response and exhibits the lowest possible tendency toward drift. The electrolyte of the measuring cell is so configured that the reaction products of the measuring reaction do not affect subsequent measurements.