The ‘metallised membrane electrode’ is made by evaporating or sputtering metal on to a non-porous but gas-permeable polymer membrane. The ‘amperostat’ system consists of an electrochemical cell with two such electrodes and an auxiliary electrode in a control circuit. The system minimise the effect of temperature on the background current. It also allows the electrocatalytic activation of carbon monoxide sensors to be controlled, and the activity to be maintained. Such sensors, made at SMRE or by collaborators, have given useful results in the field and in the laboratory over a number of years. Preliminary results suggest that, with ancillary equipment, they can make a significant contribution to the measurement of carbon monoxide at concentrations of 10 ppm upwards, and in a wide variety of gases: internal combustion exhaust gases, flue gases, cigarette smoke, exhaled air, polluted air, and air from environments that are subject to spontaneous combustion.