Reactions in thin film and diffusion barriers are important for applications such as protective coatings, electrical contact and interconnections. In this work the effect of a barrier on the kinetics of the formation for a single phase by reactive diffusion is investigated from both experimental and modeling point of view. Two types of diffusion barriers are studied: (i) a thin layer of W deposited between a Ni film and the Si substrate and (ii) Ni alloy films, Ni(1%W) and Ni(5%Pt), that form a diffusion barrier during the reaction with the Si substrate. The effect of the barriers on the kinetics of -Ni2Si formation is determined by in situ X ray diffraction and compared to models that explain the kinetics slowdown induced by both types of barrier. A linear parabolic growth is found for the deposited barrier with an increasing linear contribution for increasing barrier thickness. On the contrary, the growth is mainly parabolic for the barrier formed by reaction between an alloy film and the substrate. The permeability of the two types of barrier are determined and discussed. The developed models fit well with the dedicated model experiments, leading to a better understanding of the barrier effect on the reactive diffusion and allowing to predict the barrier behaviour in various applications.