The kinetics of the reaction between persulphate and ferrocyanide have been studied at 35°. Using glycine buffers of constant ionic strength (0·1M) in the pH range 8·2–11·6, the reaction rate is found to decrease with increase in pH, and tends towards a constant value above pH 10·8. In glycine buffers above pH 10·8, the order, as found from initial-rate measurements, is 1 with respect to persulphate and 2 with respect to ferrocyanide. In the presence of added potassium hydroxide, and also in a borate buffer (pH 11·1), the reaction is first-order in ferrocyanide. The relative effects of added alkali-metal ions in enhancing the rate lie in the order K+ > Na+ > Li+. Magnesium chloride is more effective, but this may partly rise from a change in pH. Added ferricyanide decreases the rate, but this is not due to the occurrence of the reverse reaction. Potassium chloride, potassium sulphate, and potassium citrate at the same equivalent concentration, in the presence of 0·001M-potassium hydroxide, give the same rate. On allowing for ion association, the data obtained in the presence of added potassium hydroxide fit a reaction scheme involving KS2O8– and KFe(CN)63–, as well as a postulate that a potassium ion is a necessary member of the activated complex.