We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P-700* to F-A/B iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem 1: (1) P-700* -> A(1)-F-x (< 10 ns) and (2) A(1)-F-x -> F-A/B(-) (20-200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (< 10 ns) with large negative enthalpy (-0.8 +/- 0.1 eV) and large volume change (-23 +/- 2 A(3)), which are assigned to the P-700* -> A(1)-F-x step, and (2) a component with similar to 200 ns lifetime, which has a positive enthalpy (+0.4 +/- 0.2 eV) and a small volume change (-3 +/- 2 A(3)) that are attributed to the A(1)-F-x -> F-A/B_ step. For the fast reaction using the redox potentials of A(1)F(x) (-0.67 V) and P-700 (+0.45 V) and the energy of P-700* (1.77 eV), the free energy change for the P-700* -> A(1)-F-x step is -0.63 eV, and thus the entropy change (T Delta S, T = 25 degrees C) is -0.2 +/- 0.3 eV. For the slow reaction, A(1)-F-x -> F-A/B(-), taking the free energy of -0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283-310], the entropy change (T Delta S) is positive, +0.54 +/-0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A(-)F(x) to F-A/B(-) step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.