An oxy-heme complex, the heme-superoxo species (tetrahydrofuran)(F-8)Fe-III-(O-2(center dot-)) (2) (F-8 = an ortho-difluoro substituted tetraarylporphyrinate), reacts with nitrogen monoxide (center dot NO; nitric oxide) to produce a nitrato-iron(III) compound (F-8)Fe-III-(NO3-) (3) (X-ray). The chemistry mimics the action of center dot NO Dioxygenases (NODS), microbial and mammalian heme proteins which facilitate center dot NO detoxification/homeostasis. A peroxynitrite intermediate complex is implicated; if 2,4-di-tert-butylphenol is added prior to center dot NO reaction with 2, o-nitration occurs giving 2,4-di-tert-butyl-6-nitrophenol. The iron product is (F-8)Fe-III-(OH) (4). The results suggest that heme/O-2/center dot NO chemistry may lead to peroxynitrite leakage and/or exogenous substrate oxidative/nitrative reactivity.