Time-resolved laser-induced optoacoustic spectroscopy was used for the determination of the enthalpy, Delta H-T, and structural volume changes, Delta V-T, concomitant with triplet state formation upon excitation of meso-tetrakis( 4-sulfonatophenyl) porphyrin, TSPP4-, as well as with the triplet state electron-transfer ( ET) quenching by benzoquinone, BQ ( Delta H-R and Delta V-R). The values of Delta H-T and Delta V-T for (TSPP4-)-T-3 formation in the presence of different cations ( Li+, Na+, K+, NH4+, and Cs+) correlated with each other and afforded a value of Delta(T)G = 140 +/- 20 kJ mol(-1), equal to the value of ET at 77 K, but much larger than the Delta H-T values in solution at room temperature, due to the large entropic factor in solution. The influence of the cations on Delta H-T and Delta V-T ( a contraction ranging from 5.4 to 3.8 cm(3) mol(-1)) is attributed to changing chromophore-water interactions in the ground and triplet states. Upon quenching of 3TSPP4- by BQ, the quantum yield of free radical formation, R = 0.66 +/- 0.04, is the same in the solutions of the five cations. The values of Delta H-R and Delta(RV) are small and have a large error. The energy level of the free radicals formed is thus very similar to that of (TSPP4-)-T-3. T Delta S-R and X = T Delta S-R/Delta V-R, i.e., the structural volume change-normalized entropy change for free radicals formation, were derived using average values of Delta H-R and Delta V-R together with the calculated Delta(R)G. The measured Marcus reorganization energy, I, and X fall into the I vs X linear dependence we previously found for the radical formation upon ET quenching of triplet flavins ( (FMN)-F-3 and (3)FAD) by amines and amino acids. Thus, X = T Delta S-R/Delta V-R in aqueous solutions is a property of the particular donor-acceptor pair linearly correlated to the corresponding Marcus reorganization energy. The value of X is much larger than the predicted value applying the electrostriction concepts in view of the noncontinuum nature of the aqueous solutions.