The kinetics of mercury(II)-catalyzed substitution of cyanide ligand in hexacyanoruthenate(II) by pyrazine (Pz) has been investigated spectrophotometrically at 370 nm in aqueous medium. The reaction exhibits first-order dependence on [Pz] at low concentrations, then reaches a maximum value, and finally decreases at high [Pz]. The reaction has a variable-order dependence in [Ru(CN)₆ ⁴⁻], unity at lower [Ru(CN)₆ ⁴⁻], and fractional order, not tending to zero order at higher [Ru(CN)₆ ⁴⁻]. The effects of pH, ionic strength, concentration of catalyst, and temperature variations have been studied. The activation parameters for the reaction were calculated. We propose a solvent assisted interchange dissociative (I _d) mechanism for the reaction.