A reversible carboxylate shift has been observed in a mu-oxo diferric complex in solution by UV-vis-NIR and FTIR spectroscopy triggered by the addition of a base or an acid. A terminal acetate decoordinates upon the addition of a proton, resulting in a shift of the remaining terminal acetato to a mu-eta(1) :eta(1) bridge. The addition of a base restores the original structure containing only terminal acetates. The implications for metalloenzymes with carboxylate-bridged nonheme diiron active sites are discussed.