The thallium(III)-catalysed aquations of Co(NH₃)₅Cl²⁺ and cis- and trans-Co(en)₂Cl₂⁺(first chloride) show a first-order dependence on [Tl^III] over a wide range. There is no evidence in the case of the cis-complex for the formation of a stable binuclear adduct as in the corresponding mercury(II)-catalysed aquation. Enthalpies of activation are 3·5–6·1 kcal mol^–1 less and entropies of activation 4–5 e.u. less than those observed for the uncatalysed reactions. Rate constants for the aquation of Co(NH₃)₅Cl²⁺ remain essentially unchanged as the [H⁺] is decreased from 2·0M to 0·3M(the amount of TlOH²⁺ increases from 4 to 21%) and TlOH²⁺ appears to be about as effective a catalyst as Tl³⁺. In each case Hg²⁺ is more effective than Tl³⁺ and with Co(NH₃)₅Cl²⁺(at 35°) HgCl⁺(0·59 | mol^–1 s^–1)∼ Hg²⁺(0·55) > Tl³⁺(0·034) > TlCl²⁺(0·012). The charge on the catalyst appears to be important in the determination of this order of effectiveness.