An electrode structure for air-fuel ratio detecting, including a grate-like or perforated metal plate coated with a layer of alumina which is coated with a layer of platinum which is in electrical contact with a solid electrolyte which is formed for contacting both exhaust gases and the ambient atmosphere, so that when the platinum is exposed to engine exhaust gases catalytic action of the platinum lowers the oxygen partial pressure at the electrolyte surface to its equilibrium concentration causing an abrupt change in electromotive force detectible between a positive electrode in electrical contact with the solid electrolyte, and the grate electrode.