The EPR and ENDOR characteristics of the intermediate electron acceptor radical anion I -· in Photosystem II (PS II) are shown to be identical in membrane particles and in the D 1D 2 cytochrome b-559 complex (Nanba, O. and Satoh, K. (1987) Proc. Natl. Acad. Sci. USA 84, 109–112). These findings provide further evidence that the D 1D 2 complex is the reaction center of PS II and show that the pheophytin binding site is intact. A hydrogen bond between I -· and the protein (GLU D1-130) is postulated on the basis of D 2O exchange experiments. The ENDOR data of I -· and of the pheophytin a radical anion in different organic solvents are compared and the observed differences are related to structural changes of the molecule on the basis of molecular orbital calculations (RHF-INDO/SP). The importance of the orientation of the vinyl group (attached to ring I) on electron transfer is discussed.