Stochastic approaches have been widely employed for simulating the dynamics of complex reacting systems. These approaches allow the structure of a complex mechanism and its dynamics to be analysed easily, since evolution is discretized and the main reaction paths can be clearly identified. Other features of the mechanism, such as closed reaction sequences and key species, may be identified by statistical analysis of the sequence of steps. The technique is explained by the application to a standard mechanism for homogeneous NOx removal through NH3 (Thermal DeNO(x), including 73 reversible reactions. NO reduction is shown to take place thanks to a few reaction cycles driven by a limited number of key radicals.. i.e. H, OH, O, NH2, N2H, HNO.