The electron-transfer reaction between the erythrosin dianion triplet, Er-3(2-), and Mo(CN)(8)(4-) was studied by laser-induced optoacoustic spectroscopy in the presence of various cations (Li+, Na+, K+, and Cs+, all at 4.4 x 10(-2) M analytical concentration). The buffer was in each case the corresponding H3BO3/B(OH)(4)(-), M+ (pH 9.18 at 20 degreesC). The enthalpy level of the triplet Er-3(2-) is DeltaH(1) = 180 +/- 10 kJ mol(-1) independent of the cation. The structural volume change for Er-3(2-) formation is DeltaV(1) = ca. 2 mL mol(-1), attributed to intrinsic changes upon its formation. The formation of the redox products Er3- + Mo(CN)(8)(3-) also leads to an expansion, DeltaV(2), between 12 mL mol(-1) for Li+ and 8 mL mol(-1) for Cs+, which linearly correlates with the heat released during the radical formation step. The correlation is interpreted in terms of an enthalpy-entropy compensation effect, due to the strong influence of the cations on the H-bond water network, also reflected in the correlation between DeltaV(2) and the literature values for the ability of the cations to organize the water structure. The entropic term for the formation of the free radicals upon triplet quenching thus originates in the water network rearrangement in the course of the electron-transfer process and is relatively large at room temperature.