Amperometric sensors are available for many gases in atmosphere, but not for carbon dioxide because CO₂ reduction occurs at negative potentials where interference from oxygen reduction is too severe. In this work a chemical intermediate was used, whose electrochemistry changed in the presence of CO₂, to provide the sensing system. The intermediate with the most suitable properties was a solution of copper bis(l,3-diaminopropane)2+ in aqueous chloride. The mechanism was shown to be pH based, involving the release of the aquo copper (II) ion formed when the pH of the solution decreased on contact with an atmosphere containing carbon dioxide; the Cu2+ formed reduced to the chloro complex CuCl^, at a potential positive to oxygen reduction. The chemistry was shown to be reversible on contacting the solution with atmosphere not containing CO₂. Platinum and carbon were shown to be the best electrode materials, chloride or bromide salts the optimum electrolytes. Analogous processes occurred in methanol. A wide range of other Cu(II) ligand complexes were also studied but although showing related chemistry, were less suitable than copper bis(l,3-diaminopropane)*~l~ for this application. Sensors were constructed with porous carbon or platinum cathodes and the response to CO3 found to be strongly dependent on carbon dioxide concentration (0-5%COa in air). The response time to 90% of maximum was found to be less than one minute and the stability on continued and repetitive exposure was found to be good. On removing the CO2 the output current returned to zero in less than 2 minutes. Interferences from other atmospheric gases, temperature effects and water loss from the sensor were also investigated.