Abstract: An autotitration method for electrochemically determining the concentration of a substance (S) contained in a fluid, which is arranged to flow past a generator electrode (2) and downstream thereof a detector electrode (3), said substance being capable of reacting with a species which is generated electrochemically at the generator electrode, the non-reacted portion of said species being detected electrochemically at the detector electrode. According to the invention the generator electrode current (i.sub.gen.) is controlled in response to the ratio between the detector electrode current (i.sub.det.) and the generator electrode current so as to maintain said ratio (i.sub.det. /i.sub.gen. or i.sub.gen. /i.sub.det.) at a pre-determined value (N'). Either of said currents (i.sub.gen. or i.sub.det.), which is a function of the concentration of the substance to be determined, is then measured.The invention further discloses a device for carrying out this autotitration method.