Association constants (K_ML) of ion pairs formed between a series of divalent metal ions [including magnesium, and also lanthanum(III) ions](Mⁿ⁺) and the divalent anion (L^2–) of the complex penta-ammineoxalatocobalt(III) have been calculated from spectrophotometric u.v. absorbance measurements under virtually static conditions at 50 and 60 °C with ionic strength I= 0.5 mol dm^–3. Some of these constants have also been estimated from e.m.f. measurements of a glass electrode/calomel electrode cell under the same conditions. Rate coefficients of aquation (k_obs.) of the cobalt(III) species in the metal salt solutions at 60 °C and I= 0.5 mol dm^–3 were determined from timed u.v. absorbance changes. From the k_obs. and K_ML data, rate coefficients (k₃) of the metal-oxalato ion pairs were estimated. The figures for the transition metals of the first series (manganese, cobalt, nickel, copper, and zinc) and for magnesium indicate an approximate linear relationship between the K_ML and k₃ values; in terms of this, the k₃ values for cadmium, lead, and lanthanum are of a lower order. There is a broad similarity between these features and those reported by others for various metal ion catalysed reactions, such as the rates of decarboxylation of 3-oxoglutaric acid.