Variable-temperature 59 Co nuclear magnetic longitudinal relaxation and line width measurements have been performed on dilute aqueous solutions of 12 octahedral cobalt(III) complexes spanning a large range of isotropic chemical shifts (-1-12 570 ppm) at two fields (4.7 and 11.7 T). A variable-concentration 59 Co longitudinal relaxation study of Na[Co(ethylenediaminetetraacetate)] was performed, and the results of this study indicated a need for the use of dilute solutions. The quadrupolar mechanism of relaxation was found to be dominant for 59 Co nuclei in diamagnetic octahedral complexes in aqueous solution, and values of the quadrupolar coupling constants in all 12 complexes have been determined. At temperatures above 323 K, the spin-rotation relaxation mechanism contributes to the overall rate of relaxation for 59 Co in cobalt(III) complexes with high local symmetry or large moments of inertia. Previous claims that the chemical shielding anisotropy mechanism of relaxation is also a major contributor to the total 59 Co relaxation in cobalt(III) complexes are apparently unjustified. We believe that the previous incorrect conclusions were due to concentration-dependent relaxation behavior that we and others have recently observed. An approximate absolute chemical shielding scale for cobalt is also presented.