Variable-temperature Co-59 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 Co-59 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 Co-59 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 Co-59 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 Co-59 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.