10,10′-Dimethyl-9,9′,10,10′-tetrahydro-9,9′- biacridine [(ACrH)2] acts as a unique two-electron donor in the electron-transfer oxidation with various organic oxidants. The rate-determining step is electron transfer from (AcrH)2 to oxidants, followed by facile cleavage of the C(9)-C bond of (AcrH)2.+ to yield the acridinyl radical (AcrH.) and 10-methylacridinium ion (AcrH+). The second electron transfer from AcrH. to oxidants is much faster than the initial electron transfer from (AcrH)2 to oxidants. On the other hand, the corresponding monomer, 10-methyl-9,10-dihydroacridine (AcrH2), acts as a normal hydride (two electrons and proton) donor in the reactions with oxidants. Rates of electron-transfer reactions from (AcrH)2 to various inorganic and organic one-electron oxidants depend solely on the one-electron-reduction potentials of the oxidants irrespective of the size of the oxidants, indicating that (ACrH)2 acts as a novel two-electron outer-sphere electron-transfer reagent. The one-electron-oxidation potential of (AcrH)2 (vs SCE) has been evaluated as 0.62 V, which is less positive than that of the corresponding monomer (0.80 V).