Abstract The room-temperature EPR characteristics of Photosystem II reaction center preparations from spinach, pokeweed and Chlamydomonas reinhardii have been investigated. In all preparations a light-induced increase in EPR Signal II, which arises from the oxidized form of a donor to P-680 +, is observed. Spin quantitation, with potassium nitrosodisulfonate as a spin standard, demonstrates that the Signal II species, Z ⨥, is present in approx. 60% of the reaction centers. In response to a flash, the increase in Signal II spin concentration is complete within the 98 μs response time of our instrument. The decay of Z ⨥ is dependent on the composition of the particle suspension medium and is accelerated by addition of either reducing agents or lipophilic anions in a process which is first order in these reagents. Comparison of these results with optical data reported previously (Diner, B.A. and Bowes, J.M. (1981) in Proceedings of the 5th International Congress on Photosynthesis (Akoyunoglou, G., ed.), Vol. 3, pp. 875–883, Balaban, Philadelphia), supports the identification of Z with the P-680 + donor, D 1. From the polypeptide composition of the particles used in this study, we conclude that Z is an integral component of the reaction center and use this conclusion to construct a model for the organization of Photosystem II.