The oxidation kinetics of iron and molybdenum cyanocomplexes by peroxynitrous acid were studied by computational techniques. The rate constants of ONOOH decomposition and substrate oxidation were calculated by fitting the experimental data to the solution of the ordinary differential equations mechanism. There is a linear relationship between the rate constants which is solved by varying the initial concentration of one of the reactive species. The rate constants of the steps involved in ONOOH decomposition were also determined following the same procedure and considering a linear relationship between the rate constants. The results are consistent with the values previously found using the conventional method of integration, and they validate the prior steady state assumption.