The total structural volume (ΔVstr) and enthalpy (ΔHstr) changes associated with the formation of the triplet metal-to-ligand charge transfer (3MLCT) state were determined by laser-induced optoacoustic spectroscopy for the complexes Ru(bpy)32+, Ru(bpy)2(CN)2, and Ru(bpy)(CN)42- in aqueous solutions of 0.1 M monovalent salts. For the cyano complexes (bound to water through hydrogen bonds) the values of ΔVstr vs ΔHstr exhibited a linear relationship in the salt series, whereas for Ru(bpy)32+ the values were independent of the salt. This linear correlation is interpreted as arising from an enthalpy−entropy compensation effect of the water structure perturbed by the 0.1 M salts. Since in all cases the intrinsic energy of the 3MLCT state was unperturbed by the salts (as determined by the constancy of the absorption and emission spectra), the plots ΔHstr vs ΔVstr (the latter shown to be proportional to ΔSstr) yield the free energy for the formation of the 3MLCT state, ΔGMLCT = (123 ± 8) kJ/mol for Ru(bpy)2(CN)2 and (67 ± 25) kJ/mol for Ru(bpy)(CN)42-. The higher stability of the 3MLCT state of Ru(bpy)(CN)42- is due to the larger entropic factor which originates in the high flexibility photoinduced in its 3MLCT state by the loosening of four CN-bound water molecules, rather than only two in Ru(bpy)2(CN)2. The correlation ΔVstr vs ΔSstr (a consequence of the correlation between the environment reorganization parameters ΔVsol and ΔSsol) finds support in the proportionality between ΔVstr of the cyano complexes and the calculated water-structuring entropy of the salts, ΔS°str(salt). Water structuring salts [negative ΔS°str(salt) values] afforded a larger ΔVstr, due to the extension of the water network, opposite to structure breaking salts [positive ΔS°str(salt) values] which yielded smaller ΔVstr values. For Ru(bpy)(CN)42- the linear dependence had twice as large a slope as for Ru(bpy)2(CN)2, reflecting the difference in the number of hydrogen-bound CN groups.