The kinetics of the reaction of nitric oxide with the ruthenium porphyrin carbonyl complexes Ru(P)(CO) (P octaethylporphyrin (OEP) or tetra-m-tolylporphine (TmTP)) to give the respective nitrosyl nitrito complexes Ru(P)(NO)(ONO) plus N2O were studied by stopped flow spectrophotometric techniques. Temporal spectral changes proceed in two stages via formation of an easily discernible intermediate X. The rates of both stages are first order in ruthenium concentration. In the NO concentration range 40-400 mu M, the rate law of the first stage is first order in [NO], but the slower second stage displays a rate law second order in [NO]. In the presence of some added CO, it is possible to demonstrate that the equilibrium between Ru(P)(CO) and X has a second-order dependence on [NO]. As a consequence, it is concluded that X is the dinitrosyl complex Ru(P)(NO)(2). No other intermediates develop sufficient concentrations to be observable by the stopped flow method. The kinetics behavior of this system is interpreted in terms of a rate-limiting step in the first stage involving CO dissociation from a reactive intermediate Ru(P)(CO)(NO) formed in very low concentrations by the reversible addition of NO to Ru(P)(CO). Several hypothetical mechanisms for the second stage are considered, and it is concluded that the evidence is most consistent with a pathway where the Ru center serves as a template to assemble an oligomer of NO which decomposes to nitrous oxide plus coordinated nitrite.