Kinetics of the aquation and radioactive chloride exchange reactions of trans-[Co(L)Cl₂]⁺, (L =meso- or, L′=rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane), have been investigated over a range of temperature. The studies confirm that the two co-ordinated chlorides in the L complex are kinetically equivalent with ΔH^‡= 25·6 ± 0·7 kcal mol^–1 and ΔS^‡= 13 ± 2 cal deg^–1 mol^–1, whereas they are kinetically different in the L′ complex with ΔH₁^‡= 28·3 ± 1 kcal mol^–1, ΔS₁^‡= 25 ± 5 cal deg^–1 mol^–1, and ΔH₂^‡= 39·5 ± 1 kcal mol^–1, ΔS₂^‡= 58 ± 5 cal deg^–1 mol^–1. The observation that k₁(L′) > k(L) > k₂(L′) > k(1,4,8,11-tetra-azacyclotetradecane), where k represents the appropriate first-order rate constant for the release of the co-ordinated chloride, is satisfactorily explained in terms of the steric acceleration of unimolecular reactions of these complexes. The relatively large values of ΔS^‡ found for these systems are taken to imply a relaxation of the sterically crowded complexes in the transition state.