Abstract: A system adapted to continuously and accurately measure the concentration of chlorine dioxide dissolved in liquid, the system including a sensor immersible in the liquid being tested. The sensor is constituted by a noble-metal measuring electrode, an oxidizable-metal counter electrode and an electrolyte which together define an electrochemical cell whose output current depends on the amount of chlorine dioxide passing into the cell through a diffusion membrane permeable to chlorine dioxide but not to the liquid. The chlorine dioxide within the cell is electrochemically reduced at the surface of the measuring electrode to generate a current proportional to its concentration, the counter-electrode being oxidized to complete the cell reaction.