Photoacoustic measurements using a broad frequency band hydrophone were carried out in photosynthetic reaction centers (RC) isolated from Rhodobacter sphaeroides R-26 purple bacteria. Data were extracted on enthalpy and volume changes accompanying the primary steps after excitation in the range of 0–500 μs aimed at further characterizing the thermodynamic properties of the RC. Quinone titration showed that the volume contraction accompanying the electron transport is sensitive to the molecular species occupying the secondary quinone site. ΔV_M = 14.4, 7.7 and 4.3 cm³ molar volume contractions were calculated from the measured parameters for 1, 2 and 0.07 quinone/RC after light excitation. Comparing the enthalpy changes (ΔH) to the Gibbs free energy data in the literature, a rather large (26%) entropic contribution to the free energy changes (ΔG) is estimated for the P*Q_AQ_B → P Q_A⁻Q_B electron transport (where Q_A and Q_B represent primary and secondary quinones, respectively). This is in contrast to previous estimations that ΔG = ΔH in these processes. On the other hand, only a small (4%) entropic contribution to the ΔG of the P*Q_AQ_B → P Q_AQ_B⁻ process is estimated, in agreement with the literature data. Our results are in good agreement with the data obtained earlier (Edens et al. [2000] J. Am. Chem. Soc. 122, 1479–1485).