The unique active site of flavo-diiron proteins (FDPs) consists of a nonheme diiron-carboxylate site proximal to a flavin mononucleotide (FMN) cofactor. FDPs serve as the terminal components for reductive scavenging of dioxygen or nitric oxide to combat oxidative or nitrosative stress in bacteria, archaea, and some protozoan parasites. Nitric oxide is reduced to nitrous oxide by the four-electron reduced (FMNH2-(FeFeII)-Fe-II) active site. In order to clarify the nitric oxide reductase mechanism, we undertook a multispectroscopic presteady-state investigation, including the first Mossbauer spectroscopic characterization of diiron redox intermediates in FDPs. A new transient intermediate was detected and determined to be an antiferromagnetically coupled diferrous-dinitrosyl (S = 0, [{FeNO}(7)](2)) species. This species has an exchange energy, J >= 40 cm(-1) (JS(1), degrees S-2), which is consistent with a hydroxo or oxo bridge between the two irons. The results show that the nitric oxide reductase reaction proceeds through successive formation of diferrous-mononitrosyl (S = 1/2, Fe-II{FeNO}(7)) and the S = 0 diferrous-dinitrosyl species. In the rate-determining process, the diferrous-dinitrosyl converts to diferric ((FeFIII)-F-III) and by inference N2O. The proximal FMNH2 then rapidly rereduces the diferric site to diferrous ((FeFeII)-Fe-II), which can undergo a second 2NO -> N2O turnover. This pathway is consistent with previous results on the same deflavinated and flavinated FDP, which detected N2O as a product (Hayashi et al. Biochemistry 2010, 49, 7040). Our results do not support other proposed mechanisms, which proceed either via "super-reduction" of [{FeNO}(7)](2) by FMNH2 or through Fe-II{FeNO}(7) directly to a diferric-hyponitrite intermediate. The results indicate that an S = 0 [{FeNO}(7)}](2) complex is a proximal precursor to N-N bond formation and N-O bond cleavage to give N2O and that this conversion can occur without redox participation of the FMN cofactor.