We have measured with precision the temperature and pressure dependence of the Co59 nuclear resonance chemical shift (σ) in the octahedral cobalt complexes Co(NH3)63+, Co(CN) 68-, and Co(NO2)63- in aqueous solution. The pressure and temperature range was 1 to 10 000 kg/cm 2 and 3° to 80°C, respectively. From the pressure dependence of a and a theoretical determination of the compressibility of the complexes we have deduced the dependence of the crystal-field splitting on the cobalt-ligand distance. A theory for the explicit temperature dependence of the chemical shift is presented. The fundamental parameters which enter the theory are the IS normal mode frequencies when the cobalt electrons are in the ground state and when they are in the excited state. While the former quantities are known from the vibrational spectra of these complexes, the latter are unknown. By fitting the theory to the experimental data, one concludes that on the average the vibration frequencies in the 1T1g excited electronic state must be smaller than those in the ground state by about 30% for the cyanide and nitrite complexes and by about 20% for the ammonia complex. A nonlinear dependence of a on the temperature is predicted by the theory and observed experimentally.