Early EPR and ENDOR measurements of PSI reported a fully symmetrical spindensity distribution between both monomers in P700. However, more recent work proposed a large range of asymmetric spin-density distributions ranging from 75%/25% to about 91%/9% in favor of P B and even completely localized at P B distributions. Thus, ESEEM experiments by Davis et al. proposed that the inner pair in spinach PSI is either a dimer with the spin-density ratio from 75%/25% to 80%/20% or a single monomer. The ratio of 75%/25% was confirmed by Rigby et al. using ENDOR and special triple (ST) spectroscopy. Later, 2D-ESEEM measurements for 15 N-labeled P700 and ESEEM analysis of single crystals of PSI reported spin-density distributions of about 90%/10% and 87.5%/12.5%, respectively. In an extensive study by Mac et al. including ESEEM analysis, isotopic substitution, and numerical simulations of the ESEEM data, it was shown that the spin density of P700 + is completely localized at one Chl a molecule, which interacts with the surrounding protein environment via axial ligation or hydrogen bonds. The complete localization was also reported by Lubitz and co-workers employing ENDOR spectroscopy and site-directed mutation on P700.