The kinetics and mechanism of the mercury(II)-catalysed ligand substitution reaction between potassium hexacyanoferrate(II) and 2-methylpyrazine (2-Mepz), in potassium hydrogen phthalate buffer of pH 3.50 ± 0.02 at 25.0 ± 0.1°C and constant ionic strength of 0.1 M, were studied spectrophotometrically by measuring the absorbance of the yellow-coloured product at 447 nm (k max ). The reaction was studied under pseudofirst-order conditions using excess of 2-Mepz. The reaction was found to obey first-order dependence in concentration of both [Fe(CN) 6 ] 4-and 2-Mepz. At higher concentrations of 2-Mepz, the effect of [2-Mepz] levels off. A complex behaviour is observed on varying [Hg 2? ]. The effect of pH on the reaction rate was also analysed. The effects of dielectric constant and water content of the reaction medium have been interpreted in terms of the formation of a polar activated complex and suggest an I d mechanism. The inhibitory effect of 2,3-dimercaptopropanol on the reaction was also studied and explained in terms of binding of this species to the catalyst.