The synthesis and characterisation of cis-[CoCl₂L]Cl and three diastereoisomeric trans-[CoCl₂L]ClO₄ complexes [designated (B), (C), and (D)] are described (L = C-rac-5,12-dimethyl-1,4,8,11-tetra-azacyclotetradecane). Ten diastereoisomeric trans-[CoCl₂L]⁺ complexes are in principle possible. Isomer (B) is pale green, is aquated without isomerisation (k_aq.²⁵= 4.5 × 10^–5 s^–1), and has the trans III (RSSR) configuration of the chiral nitrogen centres, with one equatorial and one axial methyl substituent (confirmed by ¹H n.m.r. measurements). This isomer is comparable with the ‘aquation-inert’[CoCl₂-(cyclam)]⁺ isomer (cyclam = 1,4,8,11-tetra-azacyclotetradecane). Isomer (C) is bright green, is aquated more rapidly than isomer (B)(k_aq.²⁵= 6.5 × 10^–4 s^–1) to give a green chloroaqua-complex indicating retention of configuration, and could have a configuration with two equatorial methyl groups. Isomer (D), which is deep green, is prepared by isomerisation of cis-[CoCl₂L]Cl in HCl solution. Isomer (D) is aquated quite rapidly (k_aq.²⁵= 9.0 × 10^–4 s^–1) with isomerisation to give a violet chloroaqua-species, and probably has a configuration with two equatorial methyl groups and two ‘twist-boat’ six-membered chelate rings. Activation parameters for aquation of the cis and trans complexes have been determined, and their relative reactivites are discussed. The ability of the macrocyclic ligand to fold in the transition state of a dissociative aquation reaction is shown to be important in determining the reactivities of the complexes.