For the first time, the effect of pressure on proton-coupled electron-transfer reactions of two selected seven-coordinate Fe-III/II(H2L)(H2O)(2) systems [where H2L = 2,6-diacetylpyridine-bis(semicarbazone) and 2,6-diacetylpyridine-bis(semioxamazide), respectively] was examined. The acid-base equilibria of the different Fe-III/II systems were investigated by spectrophotometric, potentiometric, and electrochemical titrations. On the basis of the obtained species distributions, the pH intervals in which the different protonated forms of the two studied systems exist were defined. In different pH ranges, a different number of protons (from 0 to 3 protons per electron) can be transferred during the redox process, which affects the change in the overall charge on the complexes. For all the different protonation forms of the studied complexes, the change in the redox potentials with pressure was measured and the redox reaction volume was obtained by high-pressure cyclic voltammetry. The results show that in the case of proton-coupled electron transfer, the reaction volume for the neutralization of protons contributes to the overall reaction volume. A linear correlation between Delta z(2) (change in the square of the charge) and the overall reaction volume of the complexes upon reduction, Delta V-complex, was found. The average value of the intrinsic volume change for the selected seven-coordinate iron complexes was estimated from the intercept of the Plot Of Delta Vc(omplex)(0) versus Delta z(2) to be 9.2 +/- 0.7 cm(3) mol(-1). For the combined redox and protonation processes, the data are discussed in terms of linear correlations between Delta z(2) and the redox and neutralization reaction volumes reported in the literature.