The title MnIV complex, [Mn(LH2)3] 4+ (LH2 = biguanide = H2NC(=NH)NHC(=NH)NH 2), an authentic two-electron oxidant, quantitatively oxidizes hydrazine (H2NNH2) to dinitrogen in the pH interval 2.00-3.50. The net four-electron oxidation of hydrazine is provided by two MnIV as established by stoichiometric studies. The overall reaction is composed of two parallel paths: [Mn(LH2)3]4+ + N2H5+ k1 → products [Mn(LH2)2(LH)]3+ + N2H 5+ k2 → products The values of k 1 and k2 (in M-1 S-1) are, respectively, (5.50 ± 0.6) · 10-3 and 11.4 ± 1.5 at 25.0° and I = 1.0M (NaNO3). The order k2 ≫ k1 is unusual as protonated oxidants are generally more reactive than their conjugate bases. The remarkably superior reactivity of the conjugate base of the oxidant in oxidizing N2H5+ may be at least in part due to an overall positive-charge reduction in [Mn(LH 2)2(LH)]3+ as compared to [Mn(LH 2)3]4+ that favors reaction with positively charged N2H5+, and more plausibly, a change in the mechanism is responsible for this result as not k1 but k 2 is substantially lowered in D2O media indicating that proton motion is coupled with electron transfer in the k2 path. The rate-determining step appears to be a one-electron transfer from N 2H5+ to MnIV. © 2005 Verlag Helvetica Chimica Acta AG.