The paper extends earlier studies on the S2YZ intermediate that is trapped by illumination in the temperature range 77K to 190K of untreated samples poised in the S 2 …Q A state. X-band EPR experiments on untreated and glycerol (50% v/v) treated samples at 10K indicate that the intermediate consists of two components. A wide one with a splitting of ca 170G, and a narrow one characterized by a splitting of ca 120G (untreated), or 124G (glycerol-treated samples). Lower temperatures of illumination in the above temperature range favor the wide component, which at 10K decays faster than the narrow one. Re-illumination at 10K after decay of the signal trapped at 77–190K induces only the narrow component. Rapid scan experiments in the temperature range 77–190K reveal high resolution spectra of the isolated tyz Z radical and no evidence of alternative radicals. The two split signals are accordingly assigned to different conformations of the S2YZ intermediate A point-dipole simulation of the spectra yields “effective distances” between the spin densities of YZ and the Mn 4 Ca center of 5.7Å for the wide and 6.4Å for the narrow component. The results are discussed on the basis of a molecular model assuming two sequential proton transfers during oxidation of tyr Z. The wide component is assigned to a transient S2YZ conformation, that forms during the primary proton transfer.