Abstract: The present invention provides a sensor, preferably an electrochemical sensor, for detecting acidic gases, e.g. carbon dioxide. The electrochemical sensor has a sensing electrode (20, 22), a counter electrode (37) and a body of electrolyte containing a complex, e.g. copper(II) bis(propanediamine), which has one or more ligands that is displaceable by the presence of the acidic gas in the electrolyte to form a modified complex which can be electrochemically reacted at the sensing electrode at a potential at which the unmodified complex is unreactive. The acidic gas diffuses into the electrolyte through a porous membrane. The modified complex can also be detected colorimetrically because it absorbs light at a different wavelength to the unmodified complex.