The reaction kinetics of a set of ruthenium nitrosyl complexes, {(X)(5)MNO}(n), containing different coligands X (polypyridines, NH3, EDTA, pz, and py) with cysteine (excess conditions), were studied by UV-vis spectrophotometry, using stopped-flow techniques, at an appropriate pH, in the range 3-10, and T = 25 degrees C. The selection of coligands afforded a redox-potential range from -0.3 to +0.5 V (vs Ag/AgCl) for the NO+/NO bound couples. Two intermediates were detected. The first one, I-1, appears in the range 410-470 nm for the different complexes and is proposed to be a 1:1 adduct, with the S atom of the cysteinate nucleophile bound to the N atom of nitrosyl. The adduct formation step of I-1 is an equilibrium, and the kinetic rate constants for the formation and dissociation of the corresponding adducts were determined by studying the cysteine-concentration dependence of the formation rates. The second intermediate, I-2, was detected through the decay of I-1, with a maximum absorbance at ca. 380 nm. From similar kinetic results and analyses, we propose that a second cysteinate adds to I-1 to form I-2. By plotting In k(1(RS)())(-) and In k(2(RS)())(-) for the first and second adduct formation steps, respectively, against the redox potentials of the NO+/NO couples, linear free energy plots are obtained, as previously observed with OH- as a nucleophile. The addition rates for both processes increase with the nitrosyl redox potentials, and this reflects a more positive charge at the electrophilic N atom. In a third step, the I-2 adducts decay to form the corresponding Ru-aqua complexes, with the release of N2O and formation of cystine, implying a two-electron process for the overall nitrosyl reduction. This is in contrast with the behavior of nitroprusside ([Fe(CN)(5)NO](2-); NP), which always yields the one-electron reduction product, [Fe(CN)(5)NO](3-), either under substoichiometric or in excess-cysteine conditions.