Transient absorption spectroscopy has been used to study sub-picosecond energy transfer processes in isolated photosystem II (PS II) reaction centers. As reported previously [Durrant, J. R.; et al. Proc, Natl. Acad. Sci. U.S.A. 1992, 89, 11632-11636], using long wavelength (694 nm) excitation, spectral evolution of the isotropic Q(y) band bleach/stimulated emission is dominated by energy transfer processes with a 100 +/- 50 fs time constant. In contrast, depolarization of this signal occurs with a time constant of 400 +/- 30 fs, from an initial anisotropy of similar to 0.4 to a value of similar to 0.15 at 1.5 ps. This decay of the anisotropy is attributed to energy transfer between at least two degenerate states contributing to reaction center absorption circa 680 nm, with these states having approximately orthogonal transition dipoles. The transient anisotropy barely changes between 1.5 and 60 ps, indicating that under these excitation conditions equilibration of the excitation energy between reaction center excited states occurs on a sub-picosecond time scale. Transient data collected for pheophytin Q(x) absorption bands indicate that pheophytin molecules are included in the 100 fs equilibration process. These results are discussed in the context of the PS II multimer model and are shown to be in good agreement with this model.