A series of (μ-oxo)bis(μ-acetato)diruthenium(III) complexes containing two pyridine (py) ligands and varied N-heterocyclic ligands in the positions trans and cis to μ-O, respectively, have been prepared to study py/py-d 5 exchange reactions using 1 H NMR spectroscopy. The diruthenium(III) complexes under investigation are [Ru 2 (μ-O)(μ-CH 3 COO) 2 (py) 6 ](PF 6 ) 2 (1), [Ru 2 (μ-O)(μ-CH 3 COO) 2 (bpy) 2 (py) 2 ](PF 6 ) 2 (2), [Ru 2 (μ-O)(μ-CH 3 COO) 2 (acpy) 4 (py) 2 ](FF 6 ) 2 (3), and [Ru 2 (μ-O)(μ-CH 3 COO) 2 (dmbpy) 2 (py) 2 ](PF 6 ) 2 (4), where bpy=2,2'-bipyridine, acpy=4-acetylpyridine, and dmbpy=4,4'-dimethyl-2,2'-bipyridine. Pseudo-first order rate constants for the ligand-exchange reactions are 10 - 6 -10 - 5 s - 1 for 1-4 in CD 3 CN at 298 K. It is found that the rate of the py/py-d 5 exchange reactions is controlled by the electronic nature of the cis-oriented ancillary ligands, while the exchange mechanisms are tuned principally by the ligand steric factors. The activation parameters (ΔH and ΔS ) indicate that exchange reactions proceed through the dissociative (D) or the interchange dissociative (I d ) mechanism for 1 and 3. Negative ΔS values observed for 2 and 4 suggest a significant contribution of incoming ligands to the exchange pathway. The kinetic and thermodynamic parameters for the diruthenium series and the corresponding data for Ru-based oxo bridged trinuclear complexes established previously are compared and discussed.