The kinetics of the anation ofcis-diaquo(nitrilotriacetato)cobalt(III) with imidazole (imz) and benzimidazole (bimz) have been studied in the pH range 6.10–7.10 and 5.60–6.85 respectively at μ = 0.1 M and in the temperature range 40–60°C. Both reactions produce the bisubstituted product but the entry of the first ligand is rate determining in each case. The pH dependence of rate shows that the neutral ligand reacts with both the acid and the base forms of the complex, the reactivity of the base form being slightly higher in each case. The pseudo first-order rate constants show a non-linear dependency on the concentration of incoming ligand at a particular pH. A pureD (dissociative) mechanism rather thanId(dissociative interchange) is operative which is supported by previously reported results of imidazole substitution reactions.