All of the stationary points on the potential energy surface of the S --> N isomerization and aquation of the Co(NH3)(5)SCN2+ ion have been investigated with ab initio quantum chemical methods. Also the corresponding anations of the Co(NH3)(5)OH23+ ion by the N and S ends of SCN- and the substitution of thiocyanate via the D mechanism have been studied, All calculations have been performed by taking into account hydration. The most favorable reaction of Co(NH3)(5)SCN2+ is the isomerization. It is concerted, follows the I or Id mechanism, depending on the applied criteria, and proceeds via a T-shaped transition state. The equations of Co(NH3)(5)SCN2+ and Co(NH3)(5)SCN2+ and the corresponding inverse reactions, the anations, all proceed via the Id mechanism. The activation energies, calculated for the isomerization and aquation, agree with experiment, and so does the difference of the activation energies for the anations by the two donors of SCN-. This energy difference reflects the disparate nucleophilicities of the N and S ends of SCN- and shows that bond making in the transition state is significant for the Id mechanism. Isomerization and aquation are two parallel reactions which proceed via two disparate transition states. The computed activation energy for the SCN- substitution via the D mechanism is the highest, and therefore, this pathway is unlikely to operate for the isomerization and aquation of Co(NH3)(5)SCN2+. The S --> N isomerization and the SCN- substitution via the D mechanism were furthermore computed for the free ions in the gas phase: the isomerization would require a higher activation energy and follow the I, mechanism. The activation energy for the SCN- substitution via the D mechanism would be very high, because of the large electrostatic work which is required for the removal of an anion from a (formally) 3+ charged cation.