A non-isolated potentiostat suitable for the determination of electroactive species in electrically grounded metal-walled reactors is described. In these reactors, due to the large area of the container, the rest potential of the solution is virtually constant and cannot be controlled by immersing a relatively small counter electrode connected to a conventional potentiostat. Electroanalytical methods, therefore, can be performed in reactors only with an electrically well isolated potentiostat connected to a floating excitation signal generator and data processor. The non-isolated 'grounded counter electrode' potentiostat offers a better solution; the metallic container serves as a counter electrode and the potential of the working electrode is adjusted to the sum of the excitation signal and the potential of the reference electrode located near the working electrode. This approach is also very useful in systems containing shielded electrodes whose potentials cannot be controlled by conventional multi-electrode potentiostats.