The enthalpy and volume changes for the charge-transfer reaction between excited donor and ionized donor and acceptor in bacterial reaction centers were determined using pulsed photoacoustics. Excitation in the lowest absorption band of the centers at 860 nm minimized the thermal signal caused by degradation of excess energy. Knowing the free energy of this reaction, -0.86 eV, the determination of the enthalpy, -0.44 eV, fixes the entropy at 25 degrees C as about one-half (T Delta S = +0.42 eV) of the free energy for the normal ubiquinone-10 containing centers. This is a larger contribution than anticipated from previous estimates of the enthalpy. The unexpected sign of the entropy is assigned to the release of counterions from the reaction center surfaces when the charge transfer cancels the dominant opposite charges at the interfaces. The enthalpy and entropy of six reaction centers containing exchanged quinones did not correlate with their free energies. The volume contractions ranged from -28 to -42 Angstrom(3) and roughly correlated with the size of the quinone as expected from electrostriction.