The temperature dependence of electron transport through photosystern II (PSII), measured as oxygen evolution, was investigated in thylakoid membranes irradiated with white light of 450 mu mol of photons/(m(2).s). The experiments were performed in steady-state conditions at temperatures between 273 and 303 K. The results show discontinuities, or thermal transitions, in the temperature-response curves of oxygen evolution. The experimental data was examined with the Marcus theory of electron transfer modified to take into account the oxygen evolution discontinuities. For this purpose, the Gibbs free energy of activation of the electron transfer reaction, Delta G degrees, is replaced in the classical Marcus equation with the expression Delta H degrees - T Delta S degrees, where H degrees and Delta S degrees are respectively the enthalpy and entropy of activation, and T is the temperature in kelvin. The result of the derivation is a summation of j Gaussian functions, or states, OE = 69 250 Sigma(j){(V-DA(4)/lambda T)(1/2) exp[-(T-max - T)(2)/2T sigma(2)(o)]}(j) (eq 1), where OE is expressed in mu mol oxygen evolution.(mg Chl.h)(-1), and V-DA is the electronic coupling matrix element between electron donor (D) and acceptor (A) wave functions, lambda the reorganization free energy, k(B) the Boltzmann constant, T-max = (Delta H degrees + lambda)/Delta S degrees, sigma(o) = (2k(B)lambda/Delta S degrees 2)(1/2), and sigma = T-1/2 sigma(o) is the standard deviation of the Gaussian hand. The mathematical simulations revealed the presence of six thermal transitions, or Gaussian bands with maxima at 275.3, 281.2, 286.4, 291.4, 297.1, and 302.4 K. The resolution of the Gaussian bands is about 0.55 owing to multiple hand superpositions. The theoretical analyses showed that (i) the oxygen evolution in PSII is essentially dependent on V-DA(2)/(lambda T)(1/2) in the pre-exponential term of the modified Marcus equation (eq 1), and (ii) the reorganization energy, lambda, decreases exponentially with increasing temperature, and is also dependent on the electron donor-acceptor distance. It is concluded that the temperature dependence of the large enhancement of oxygen evolution observed in this work originates, at least partly, in heat-induced structural rearrangements in the photosystem II reaction center.