Abstract: An antifouling electrochemical gas sensor is disclosed in which the sensor has a cell containing two spaced apart electrodes immersed in an electrolyte and a membrane over an opening in the end of the cell which is immersed in a medium containing varying concentrations of a gas to be sensed, typically oxygen. The electrodes are also connected to an external electrical circuit which indicates the current flow between the electrodes as a result of oxidation and reduction reactions at the electrodes caused by oxygen diffusing through the membrane into the cell. The sensor has a sensing mode during which it functions to indicate the amount of oxygen in the medium. The sensor also has a cleaning mode in which the current flow across the electrodes is in an opposite direction to cause a reaction at one of the electrodes producing a biocidal gas.