Mn(III)TMPyP reacts rapidly with the toxic oxidant peroxynitrite (ONOO-)to generate an oxoMn0V) species, but Mn(I]I)TMPyP is not catalytic for ONOO- decomposition due to the slow reduction of oxoMn(IV) back to the Mn(HI) oxidation state. However, when redox-coupled with biological antioxidants that efficiently reduce oxoMn(IV), Mn(~I)TMPyP is transformed into an efficient "peroxynitrite reductase."