The dinuclear Mn-III, Mn-IV model for the reactive center in photosystem II (PS II), [(bipy)(2)Mn-III(O)(2)Mn-IV(bipy)(2)](3+) undergoes rapid reductions by ascorbate, hydroquinone, and HSO3- to 2Mn(2+) tag) in aqueous media buffered by bipy-bipyH(+). Net changes of 3e(-) occur in each case. The ascorbate system involves a (MnMnIV)-Mn-III-ascorbate complex (e = 1.4 X 10(2) M(-1) at 23 degrees C) and exhibits k(limiting) = 8.8 X 10(2) s(-1) in H2O and 2.2 X 10(2) s(-1) in D2O for the initial redox act which, in this case, is rate-determining. Complex decay profiles generated by the reactions of hydroquinone and HSO3- are in accord with multistep processes in which the intermediate Mn states and the radicals derived from the reductant (semiquinone and SO3.(-)) participate. The resulting patterns are autocatalytic in nature, with the active catalytic species appearing to be the (Mn-III)2 intermediate. Solvent kinetic isotope effects, (k(H2O)/ k(D2O)) observed when solvent H2O is replaced by D2O, lie near 4.0 for the ascorbate reduction and for the initial step in the hydroquinone reduction but approach unity for the bisulfite reduction, pointing to coupled electron and proton transfers by the organic reductants but not by HSO3-. Plots of k(obsd) VS mole fraction D2O are very nearly linear for the reduction by ascorbate, indicating that, in this case,just one proton is transferred in the rate-determining step.