The kinetics and mechanism of Hg 2+ -catalyzed substitution of cyanide ion in an octahedral hexacyanoruthenate(II) complex by nitroso-R-salt have been studied spectrophotometrically at 525 nm (λ max of the purple-red-colored complex). The reaction conditions were: temperature = 45.0 ± 0.1 • C, pH = 7.00 ± 0.02, and ionic strength (I) = 0.1 M (KCl). The reaction exhibited a first-order dependence on [nitroso-R-salt] and a variable order dependence on [Ru(CN) 6 4− ]. The initial rates were obtained from slopes of absorbance versus time plots. The rate of reaction was found to initially increase linearly with [nitroso-R-salt], and finally decrease at [nitroso-R-salt] = 3.50 × 10 −4 M. The effects of variation of pH, ionic strength, concentration of catalyst, and temperature on the reaction rate were also studied and explained in detail. The values of k 2 and activation parameters for catalyzed reaction were found to be 7.68 × 10 −4 s −1 and E a = 49.56 ± 0.091 kJ mol −1 , H = = 46.91 ± 0.036 kJ mol −1 , S = = −234.13 ± 1.12 J K −1 mol −1 , respectively. These activation parameters along with other experimental observations supported the solvent assisted interchange dissociative (I d ) mechanism for the reaction.