Rate coefficients of aquation at constant ionic strength in lithium perchlorate–perchloric acid media of a series of penta-ammine(carboxylato)cobalt(III) complexes [Co(NH₃)₅L]ⁿ⁺ have been calculated from u.v. absorbance measurements. With most of these, increases in the observed rate coefficients are not proportional to increases in the acid concentrations. The data have been used to calculate association constants of protonated forms of the complexes and the corresponding rate coefficients of aquation. The obtained association constants are 0.64 (L = acetate; I= 2.0, 40 °C), 0.1 (L =o-methoxybenzoate; I= 2.0, 40 °C), 116 (L = oxalate; first association constant, I= 0.5, 60 °C), 0.24 (L = malonate; second association constant, I= 2.0, 40 °C), 0.69 (L = succinate; second association constant, I= 2.0, 40 °C), 0.02 dm³ mol^–1(L =o-phthalate; second association constant, I= 2.0 mol dm^–3, 40 °C). With the tartrato-complex the observed rate coefficients increased linearly with increasing [HClO₄] and this is also so with the oxalato-complex for I > 0.5 mol dm^–3 but such increases are small when compared with those of the other d ica rboxylato-complexes of the present studies. U.v. spectral decreases at constant wavelength with increasing [HClO₄] under virtually static conditions have also been used to estimate the association constants of the protonated complexes. The exceptions, since no spectral decreases were found, are the o-methoxybenzoato-complex, the oxalato-form at [HClO₄] > 0.5 mol dm^–3, and the doubly protonated tartrato-complex. The values obtained are 0.68 (L = acetate; I= 2.0, 30 °C), 125 (L = oxalate; first association constant, I= 0.5, 60 °C), 0.37 (L = malonate; second association constant, I= 2.0, 40 °C), 0.67 (L = succinate; second association constant, I= 2.0, 40 °C), 0.1 dm³ mol^–1(L =o-phthalate; second association constant, I= 2.0 mol dm^–3, 40 °C). These are in reasonable agreement with the above values derived from the kinetic data and from the latter the rate coefficients of aquation of the protonated species have been estimated. There appear to be broad relationships between the rate coefficients of the protonated forms and the association constants (comparing acetate with o-methoxybenzoate and succinate with malonate). This can perhaps account for the oxalate, tartrate, and o-phthalate complexes showing little evidence of double protonation. This feature is also discussed in terms of ligand chain lengths and it is also noted that in contrast to the first association constants of the protonated dicarboxylate complexes which are large but have relatively little catalytic effect, the second association constants are small but have strong influences upon the rates of aquation.