The reaction between the trans-[RuNO(NH3)(4)P(OEt)(3)](PF6)(3) and L-cysteine (RS-) was studied over a pH range of 2.0-7.4. In this reaction, the concentrations of NO and HNO produced varied as a function of the pH of the solution. The first step of this reaction proceeded quickly [k(1) = (3.5 +/- 0.3)x10(3) M-1 s(-1), pH = 3.5, 25 degrees C] and resulted in the formation of trans-[Ru(NH3)(4)P(OEt)(3)N(O)SR](2+), which dissociated to yield trans-[Ru(NH3)(4)P(OEt)(3)NO](2+) and RS. However, trans-[Ru(NH3)(4)P(OEt)(3)N(O)SR](n-1) can react with a second L-cysteine, yielding trans-[Ru(NH3)(4)P(OEt)(3)N(O)(SR)(2)](+) [k(2) = (3.6 +/- 0.1) M(-1)s(-1), pH = 3.5, 25 degrees C]. Therefore, the trans-[Ru(NH3)(4)P(OEt)(3)NO](2+) species released NO and the trans-[Ru(NH3)(4)P(OEt)(3)N(O)(SR)(2)](n-2) species released HNO.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São Carlense, CEP: 13560-970 – São Carlos – SP, BrazilDepartamento de Química Geral e Inorgânica, Instituto de Química de Araraquara, UNESP – Universidade Estadual Paulista, P. O. Box 355, Araraquara, São Paulo 14801-970, Brazil