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The kinetics of flash-induced re-reduction of the Photosystem II (PS II) primary electron donor P 680 was studied in solution and in trehalose glassy matrices at different relative humidity. In solution, and in the re-dissolved glass, kinetics were dominated by two fast components with lifetimes in the range of 2–7 μs, which accounted for >85% of the decay. These components were ascribed to the direct electron transfer from the redox-active tyrosine Y Z to P 680 +. The minor slower components were due to charge recombination between the primary plastoquinone acceptor Q A − and P 680 +. Incorporation of the PS II complex into the trehalose glassy matrix and its successive dehydration caused a progressive increase in the lifetime of all kinetic phases, accompanied by an increase of the amplitudes of the slower phases at the expense of the faster phases. At 63% relative humidity the fast components contribution dropped to ~50%. A further dehydration of the trehalose glass did not change the lifetimes and contribution of the kinetic components. This effect was ascribed to the decrease of conformational mobility of the protein domain between Y Z and P 680 , which resulted in the inhibition of Y Z → P 680 + electron transfer in about half of the PS II population, wherein the recombination between Q A − and P 680 + occurred. The data indicate that PS II binds a larger number of water molecules as compared to PS I complexes. We conclude that our data disprove the "water replacement" hypothesis of trehalose matrix biopreservation. [Display omitted] • Photosystem II in glassy trehalose matrices at variable humidity was studied. • Forward electron transfer and charge recombination were slowed down in trehalose. • Dehydration inhibited protein conformational mobility and Y Z → P 680 electron transfer. • The data disprove "water replacement" hypothesis of trehalose matrix biopreservation. [ABSTRACT FROM AUTHOR]