The ion yields and enthalpies of reactions between the triplet states of zinc and magnesium mesoporphyrin dimethyl esters and various quinones in acetonitrile and in dimethylformamide were determined by wide band, time-resolved, pulsed photoacoustics. Sufficient information is obtained to allow a calculation of an apparent entropy from the difference between these enthalpies and the free energies or redox potentials measured under the same conditions. We assign this difference to a volume contraction, or electrostriction, on ion formation. Its magnitude agrees roughly with an estimate based on the formula of Drude and Nernst published 100 years ago.