Abstract The nucleophilic substitution reaction of S 2O 3 2− with [Ru(HaaiR′) 2(OH 2) 2](ClO 4) 2 ( 1) [HaaiR′ = 1-alkyl-2-(phenylazo)imidazole] and [Ru(ClaaiR′) 2(OH 2) 2](ClO 4) 2 ( 2) [ClaaiR′ = 1-alkyl-2-(chlorophenylazo)imidazole] [where R′ = Me( a), Et( b) or Bz( c)] in acetonitrile–water (50% v/v) medium to yield Na 2[Ru(HaaiR′) 2(S 2O 3) 2] ( 3a, 3b or 3c) and Na 2[Ru(ClaaiR′) 2(S 2O 3) 2] ( 4a, 4b or 4c) has been studied. The products were characterized by microanalytical data and spectroscopic techniques (UV–Vis, NMR and mass spectroscopy). The reaction proceeds in two consecutive steps (A → B → C); each step follows first order kinetics with respect to each complex and S 2O 3 2−, and the first step second order rate constant ( k′ 2) is greater than the second step one ( k″ 2). An increase in the π-acidity of the ligand increases the rate. Thermodynamic parameters, the standard enthalpy of activation (Δ ‡ H 0) and the standard entropy of activation (Δ ‡ S 0), have been calculated for both steps using the Eyring equation from variable temperature kinetic studies. The low Δ ‡ H 0 and large negative Δ ‡ S 0 values indicate an associative mode of activation for both aqua ligand substitution processes.