Photoacoustic calorimetry was applied to the photolysis at 266 nm of aqueous peroxodisulfate ion in the presence of bisulfite. A time-resolved analysis of the signals at variable temperature (282-295 K) yields the heat of formation (-530 kJ mol (1)) and the partial molal volume (50 mL mol (1)) of aqueous sulfite radical. The ca. 11 mL mol (1) difference between the experimental molal volumes of SO3 center dot- and the parent closed shell ion with similar intrinsic size SO3H, indicates a diverse rearrangement of the solvent molecules around each species, probably as a result of the dissimilar electronic distribution. (c) 2008 Elsevier B.V. All rights reserved.