Spin polarized transient ESR spectra of 3P68, in D,D2cytb,59-complexes of Photosystem 11 are studied as a function of temperature. The spin polarization as well as the rise time of the ESR signals are characteristic of triplet formation via recombination from the primary radical pair P&Pheo-Below 100 K the well-known spectrum of 3P6,0 is observed with zero-field splitting (zfs) parameters associated with the triplet state of monomeric chlorophyll-a (Chl). Between 100 K and 200 K the spectral pattern changes drastically and above 200 K a considerably narrowed triplet spectrum (i.e. reduced zfs parameters) is observed which suggests a delocalization of the triplet excitation. When the temperature-dependent spectra are normalized to the same triplet yield, isosbestic points are observed indicating that the spectra are weighted sums of the limiting case spectra at low and high temperature (10 K and 250 K, respectively). This is taken as evidence that a transition between two triplet states of different electronic origin occurs. The coefficients of this superposition follow a sigmoidal temperature dependence which suggests a distribution of activation energies for the triplet delocalization process in accordance with a glass-like distribution of protein matrix conformations. The narrowing of the spectrum at high temperature can be interpreted as thermally activated delocalization of the triplet excitation over at least two Chl units with different orientations with respect to the PS I1 reaction center, viz. one oriented with its plane tilted 30" with respect to the membrane plane and the other with its plane perpendicular. The similarities and differences in the cofactor arrangement of PS I1 and purple bacteria reaction center are discussed.