Analysis of Mn-55 electron spin echo-electron nuclear double resonance (ESE-ENDOR) spectra obtained on a dinuclear mixed valence Mn(III)Mn(IV) complex [di-mu-oxotetrakis(2,2’-bipyridine)dimanganese(III,IV)] (1) reveals the hyperfine and nuclear quadrupolar parameters for the spin I = 5/2 Mn-55 nucleus of both Mn(III) and Mn(IV) ions. The following parameters are obtained : for Mn(IV), A(perpendicular to) = +212 MHz, A(parallel to) = +231 MHz, P-parallel to 3e(2)Qq/40 = +2.0 MHz, and eta = 0.3, and for Mn(III), A perpendicular to = -480 MHz, A(parallel to) -360 MHz, P-parallel to = -4.5 MHz, and eta = 0.1. The Mn-55 ESE-ENDOR data obtained on the g = 2 Mn multiline EPR signal of the S-2 state of the photosystem II oxygen-evolving complex demonstrate that this EPR signal cannot arise from a dinuclear Mn(III)Mn(IV) center. The ENDOR spectra are consistent with a tetranuclear Mn cluster origin for the photosystem II multiline EPR signal.