A major challenge in understanding the mechanism of nitrogenase, the enzyme responsible for the biological fixation of N(2) to two ammonias, is to trap a nitrogenous substrate at the enzyme active site in a state that is amenable to further characterization. In the present work, a strategy is described that results in the trapping of the substrate hydrazine (H(2)N-NH(2)) as an adduct bound to the active site metal cluster of nitrogenase, and this bound adduct is characterized by EPR and ENDOR spectroscopies. Earlier work has been interpreted to indicate that nitrogenous (e.g., N(2) and hydrazine) as well as alkyne (e.g., acetylene) substrates can bind at a common FeS face of the FeMo-cofactor composed of Fe atoms 2, 3, 6, and 7. Substitution of alpha-70(Val) that resides over this FeS face by the smaller amino acid alanine was also previously shown to improve the affinity and reduction rate for hydrazine. We now show that when alpha-195(His), a putative proton donor near the active site, is substituted by glutamine in combination with substitution of alpha-70(Val) by alanine, and the resulting doubly substituted MoFe protein (alpha-70(Ala)/alpha-195(Gln)) is turned over with hydrazine as substrate, the FeMo-cofactor can be freeze-trapped in a S = (1)/(2) state in high yield ( approximately 70%). The presumed hydrazine-FeMo-cofactor adduct displays a rhombic EPR signal with g = [2.09, 2.01, 1.93]. The optimal pH for the population of this state was found to be 7.4. The EPR signal showed a Curie law temperature dependence similar to the resting state EPR signal. Mims pulsed ENDOR spectroscopy at 35 GHz using (15)N-labeled hydrazine reveals that the trapped intermediate incorporates a hydrazine-derived species bound to the FeMo-cofactor; in spectra taken at g(1) this species gives a single observed (15)N signal, A(g(1)) = 1.5 MHz.