Although the analytical determination of peroxodisulphate is state of the art, there is no well-established on-line measuring method for this substance. This paper presents an on-line monitoring device for peroxodisulphate concentration that uses a coulometric measuring method. The method is based on the redox potential control in a miniaturised electrochemical reactor. This paper presents the design of the measuring cell and describes the integration of this cell in an on-line measuring device which can be applied in a process environment. Experimental results illustrate its dynamic characteristics and accuracy.