Anion competition data are reported for the base hydrolysis (pH 8.5–9) of [(NH 3) 5CoO 3SCF 3] 2+ in 1 M NaY (Y = F −, Cl −, Br − I − and NO − 3. At 25 °C, [(NH 3) 5CoY] 2+ is formed along with [(NH 3) 5CoOH] 2+, as a direct product of the base catalyzed hydrolysis process (Y = F −, <0.2; Cl −, 7.5; Br −, 7.1:1 −, 7.5; NO − 3, 11.9%. Hydroxide ion as a nucleophile has also been examined, for both a 2+ and 3+ complex ion, by carrying out mixed anion (N − 3/OH − competition experiments at low and high [OH − (0.1 M and 1.1 M). It is concluded that OH − does not compete effectively with H 2O in the attack of the reactive reduced coordination intermediate, [(NH 3) 4(NH 2)Co] 2+ formed in the base hydrolysis reaction. In this respect, the behaviour of OH − resembles that for the isoelectronic F − ion. Both are strongly solvated by water. The extent of competition afforded by the other anions Cl −, Br −, I −, and NO − 3 is similarly accommodated, the least solvated NO − 3 ion being the best competitor. Competition by the neutral competitor NH 3 (at the 1 M level) is shown to be negligible for both a 2+ and 3+ reactant. Finally, acetate ion (1 M) competition has been determined for the base hydrolysis of [(NH 3) 5CoX] n+, using seven different leaving groups. The results remove a previous anomaly in the literature, and show that the extent of anion competition is essentially independent of the leaving group X. Finally, this study permits a direct comparison between the acid and base hydrolysis reactions of [(NH 3) 5CoX] n+. The relative nucleophilic effectiveness of Cl − (and NO − 3) towards the respective intermediates [(NH 3) 5Co] 3+ and [(NH 3) 4(NH 2)Co] 2+ believed to be involved in the two processes is evaluated. It is concluded that the different extents of anion competition reside largely in the formal charge of the intermediates. In both reactions the anion is captured, at close to but less than a diffusion controlled rate, from an ion-atmosphere determined largely by the degree of ion-association exhibited by the intermediates.