The lifetimes of metastable CO₂^- ions, formed by charge exchange of CO₂⁺ ions in alkali-metal vapour or by sputtering, have been measured by a time-of-flight technique at the storage ring ASTRID. By the method of charge exchange, two components with lifetimes in the time interval 3-8 ms can be accounted for by the data. These lifetimes, presumably representing averages over several vibrational states, are much longer than the one previously assigned to the lowest-lying electronic state of CO₂^-. The possible identity of the electronic state associated with the millisecond lifetimes is discussed.