The reduction of metal nitride to ammonia is a key step in biological and chemical nitrogen fixation. We report herein the facile reduction of a ruthenium(VI) nitrido complex [(L)Ru-VI(N)(OH2)](+) (1, L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) to [(L)Ru-III(NH3)(OH2)](+) by L-cysteine (Cys), an ubiquitous biological reductant, in aqueous solution. At pH 1.0-5.3, the reaction has the following stoichiometry: [(L)Ru-VI(N)(OH2)](+) + 3HSCH(2)CH(NH3)CO2 -> [(L)Ru-III(NH3)(OH2)](+) + 1.5(SCH2CH(NH3)CO2)(2). Kinetic studies show that at pH 1 the reaction consists of two phases, while at pH 5 there are three distinct phases. For all phases the rate law is rate = k(2)[1][Cys]. Studies on the effects of acidity indicate that both HSCH2CH(NH3+)CO2 and SCH2CH(NH3+)CO2 are kinetically active species. At pH 1, the reaction is proposed to go through [(L)Ru-IV(NHSCH2CHNH3CO2H)(OH2)](2+) (2a), [(L)Ru-III(NH2SCH2CHNH3CO2H)(OH2)](2+) (3), and [(L)Ru-IV(NH2)(OH2)](+) (4) intermediates. On the other hand, at pH around 5, the proposed intermediates are [(L)Ru-IV(NHSCH2CHNH3CO2)(OH2)](+) (2b) and [(L)Ru-IV(NH2)(OH2)](+) (4). The intermediate ruthenium(IV) sulfilamido species, [(L)Ru-IV(NHSCH2CHNH3CO2H)(OH2)](2+) (2a) and the final ruthenium(III) ammine species, [(L)Ru-III(NH3)(MeOH)](+) (5) (where H2O was replaced by MeOH) have been isolated and characterized by various spectroscopic methods.