The kinetic and mechanistic studies of Hg II catalyzed exchange of coordinated cyanide in hexacyanoferrate(II) by pyrazine (Pz) were monitored by following the appearance of the yellow complex [Fe(CN) 5 Pz] 3) at 440 nm corresponding to metal ligand charge transfer (MLCT) transitions at temp=25.0±0.1°C, pH=2.5±0.02 and ionic strength, I=0.1 M (KNO 3 ). The effect of pH, ionic strength and the concentrations of [Fe(CN) 6 ] 4) and Pz on the rate of reaction were also studied and explained. To investigate the dependency of catalytic activity of Hg II , the initial rates were determined at several concentrations of Hg II , keeping the concentration of other reactants constant. The kinetic observations suggest that the substitution follows an interchange dissociative (I d ) mechanism and proceeds via formation of a solvent-bound intermediate. The repetitive spectral scan is also provided as evidence for the exchange of cyanide ions by pyrazine in [Fe(CN) 6 ] 4) . Activation parameters have also been evaluated and provided support for the proposed mechanistic scheme.