Formation constants have been determined for the substitution of co-ordinated H₂O in aquocobalamin in aqueous solution (ionic strength usually 0·5) at room temperature by the following ligands: CH₃·CO₂^–, 4·5 l./mole; N₃^–, 7·2 × 10⁴; NO₂^–, 2·3 × 10⁵; CH₃·NC, 7 × 10⁴; SO₃^2–, 2·2 × 10⁷; thiourea, 13. No complex was detected with the ligands CO, O₂, NO, (CH₃)₂S, or Ph₃P. Additional formation constants relating to the thermodynamic trans-effect have been determined (i) when the trans-ligand is SO₃^2–, and (ii) for the substitution of H₂O by N₃^–, CH₃·NC, and SO₃^2– with different trans-ligands. It is shown that SO₃^2– is comparable with HCC^– and CH₂CH^– in its cis- and trans-effects. The absence of CO complexes and the close parallel between formation constants involving CH₃·NC and N₃^– are evidence for the absence of any significant π-bonding between the cobalt(iii) ion and the ligand.