The primary electron donor (P700) in Photosystem I (PSI) has been shown to be a dimeric chlorophyll a species. Electron magnetic resonance studies of the cation radical have clearly established that the unpaired electron is delocalized asymmetrically over this dimer; however, the extent to which this asymmetry exists remains ambiguous. Comprehensive electron nuclear double resonance (ENDOR) and electron spin−echo envelope modulation (ESEEM) experiments combined with isotopic substitution and numerical simulations have been used to determine the electronic structure of P700 + . This approach utilizes the strengths of each spectroscopy to elucidate the electron nuclear hyperfine and nuclear quadrupole coupling constants for the nitrogen nuclei in P700 + . These assignments are then confirmed by performing numerical simulations of the ESEEM data. Further confirmation of these values is obtained by performing the spin−echo experiments at multiple microwave frequencies. The same set of hyperfine and quadrupole coupling constants is used to simulate all of the ESEEM data for P700 + containing either natural abundance 14N or isotopically enriched with 15N. These simulations indicate that the unpaired spin is localized over only one of the chlorophylls that make up the special pair. The ramifications of this monomeric spin density on the function and thermodynamics of electron transfer in PSI are discussed.