The kinetics of the substitution of H 2 O on Co(III) in aquacobalamin (H 2 OCbl + , vitamin B 12a ) and in a Co(III) corrole analogue of B 12a , aqua-[10-(2-[[4-(1H-imidazol-1-yl-methyl)benzoyl]amino]phenyl)-5,15-diphenylcorrolato]-cobalt(III), [H 2 O-DPTC-Co], were determined in 80:20 MeOH/H 2 O and low ionic strength. The second order rate constant k II (25°C) for the reaction of H 2 OCbl + is 2720M −1 s −1 (ΔH ‡ =82(5) kJmol −1 and ΔS ‡ =96(19) JK −1 mol −1 ) while for [H 2 O-DPTC-Co], k II (25°C)=990M −1 s −1 (ΔH ‡ =41(1) kJmol −1 and ΔS ‡ =−50(3) JK −1 mol −1 ). It is argued that differences in ΔS ‡ are due to differential solvent electrostriction, arising from the different charges at the metal center in the two complexes. A smaller ΔH ‡ for the reaction with [H 2 O-DPTC-Co] suggests that the transition state occurs earlier along the reaction coordinate, consistent with the higher affinity of Co(III) for CN − in the corrole. On this basis only, Co(III) is more labile towards CN − in the corrole than in the corrin. However, because of differences in ΔS ‡ , H 2 OCbl + is more labile towards CN − than [H 2 O-DPTC-Co] above the isokinetic temperature of 8°C. The nature of the equatorial ligand in Co(III) macrocyclic complexes significantly affects the lability of the metal ion.