The low-spin cyanide complexes of three Co(II) carbonic anhydrases were investigated by electron paramagnetic resonance (e.p.r.) at 9 and 35GHz. Well-defined and closely axial spectra were obtained only in the absence of oxygen. Several mole equivalents of cyanide were required for complete formation of the complexes in frozen solution, although large excesses caused abstraction of the cobalt. Experiments with [(13)C]cyanide showed that the low-spin complexes contained two CN(-) groups in an environment similar to that of the in-plane ligands in [Co(CN)(5)](3-). A combined e.p.r. and spectrophotometric titration confirmed the presence of two CN(-) ligands. A 5-co-ordinate square pyramidal structure involving three protein ligands was proposed. The dicyanide complex could be oxygenated reversibly, producing a characteristic new e.p.r. spectrum. The O(2) molecule was thought to occupy the remaining octahedral metal site in a formally Co(III) species. The optical spectrum of the dicyanide lacked the prominent d-d bands of the high-spin monocyanide. Both e.p.r. and optical data indicated that the low-spin complex was formed much more fully in frozen solution than at room temperature. Differences in behaviour between the high- and low-activity enzymes suggested some variation in conformational flexibility at the metal binding site.