A new diruthenium(III) complex [RuIII2(μ-O)(μ-CH3CO2)2(bpy)2(CH3CN)2](PF6)2 [1(PF6)2, bpy = 2,2′-bipyridine] has been synthesized, and the dynamic behaviour of CH3CN at the trans-to-μ-oxido sites were investigated by 1H NMR and UV/Vis spectroscopy at 0–60 °C under 0.101–147 MPa in CH3CN. The solvent exchange in CD3CN at 5 °C gave the rate constant k = 2.43 × 10–3 s–1 and the activation parameters ΔH‡ = 124 kJ mol–1 and ΔS‡ = +150 J K–1 mol–1, which point to a dissociative mechanism (Id or D). The ligand substitution of pyridine (py) in CD3CN at 30 °C, yielding [RuIII2(μ-O)(μ-CH3CO2)2(bpy)2(py)2](PF6)2 [2a(PF6)2], gave a rate constant per site of k = 1.56 × 10–2 M–1 s–1, and the activation parameters ΔH‡ = 81 kJ mol–1 and ΔS‡ = –13 J K–1 mol–1. The substitution of various pyridine derivatives (Rpy) for CH3CN in 1(PF6)2 exhibited varying activation parameters: ΔH‡ ranged from 66 to 92 kJ mol–1, ΔS‡ ranged from –51 to +22 J K–1 mol–1, and ΔV‡ (for Rpy with R = H, 3-Ac, 4-NH2, 4-CN, 3-CN) ranged from +1.3 to +3.2 cm3 mol–1. These results suggest an interchange (I) mechanism for the substitution of Rpy, for which its strong electron-donating ability is responsible. A compensation effect is seen between ΔH‡ and ΔS‡, and both have linear relationships with the Hammett parameters. The substituent R electronically influences the strength of the interaction of Rpy with Ru center in the transition state to shift the activation mode to a dissociative (Id) or an associative (Ia) interchange.