The so-called anomalous fast diffusion in metallic alloys and semi-conductors is often analysed within an interstitial-substitutional model. The equations used for modelling these mechanisms are reaction-diffusion type whose analytical solutions are available only under drastic simplifications. The dissociative variety of this model is simulated using both finite Ε½ . Ε½ . Ε½ difference FD and Monte Carlo MC methods. In MC simulation, diffusion of different species interstitial impurities B , i . substitutional B , and vacancies V and reaction jumps occur according to the suitable probabilities which are jump s frequencies-dependent. Whereas in FD method, an implicit scheme is used to solve the system of non-linear partial differential equations. In both cases, the finite source conditions have been considered. A good agreement between results obtained by the two methods is found. On the other hand, the double-stages of simulated profiles are found to have similar Ε½ . Ε½ . shapes to those obtained experimentally in Nb Co and in GaAs Zn . The first stage is well analysed by a Gaussian function, whereas the second one is well represented by an erfc type function. Furthermore, a detailed study of the two stages leads to a qualitative agreement with Stolwijk's analysis in two limited cases where the diffusion is vacancy-or foreign interstitial-controlled. However, the effective diffusion coefficients present a quantitative departure from those obtained by Stolwijk's expressions.