59Co NMR signal of Co(H2O)63+ was observed and the chemical shift and the spin-lattice relaxation time were recorded as a function of temperature. No deviation due to an exchange with paramagnetic high-spin cobalt(III) could be observed in the chemical shift. An increase in the spin-lattice relaxation rate was observed at high temperatures. However, following the dependence of the relaxation rates on acid concentration, this increase was associated with an increased formation of Co(H2O)5OH2+ at high temperatures. An approximate upper limit for the fractin of Co(H2O)63+ which is present as high spin was estimated as f < 10-4.