The affinity of bicarboxylate ions (from oxalate to glutarate) for cobalt (II) bovine carbonic anhydrase has been investigated and compared with that of acetate and propionate. The oxalate ion shows a much greater affinity for the enzyme than acetate, whereas the other bicarboxylate ions have very little tendency to bind the enzyme. In every case, and particularly for the oxalate, the apparent affinity constants dramatically increase with decreasing pH. On the basis of the electronic spectra a five-coordinate structure is proposed for all of the above derivatives. Carbon-13 NMR data have been discussed in terms of the oxalate ion chelating the metal ion and/or interacting with the wall of the active cavity.