The formation and the decay of the triplet metal to ligand charge transfer state ((MLCT)-M-3) of ruthenium(II) bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine) (Ru(bpy)(2)(dcbpy)) were characterized using photoacoustic calorimetry. At pH 6 and 2, the 3MLCT state formation leads to a volume change of -8 mL mol(-1) and enthalpy changes of 17 kcal mol(-1) and 13 kcal mol(-1), respectively. We attribute the volume contraction to structural changes and to solvent electrostriction. At pH 4, the photoexcitation of the complex leads to an expansion of 14 mL mol(-1) and an enthalpy change of similar to119 kcal mol(-1) due to protonation of the carboxyl group in the excited state.