Magnetism plays a crucial role in the thermodynamic and kinetic properties of ferritic alloys. In fact, magnetism increases the solubility limit of Cr in Fe, inducing an asymmetrical phase diagram. Moreover, the phase transition from ferromagnetic to paramagnetic (F/P) iron alloys modifies to a large extent the system response to different environmental conditions by modification of the alloy diffusion properties. Indeed, experimental tracer diffusion coefficients deviate from an Arrhenius law during the F/P magnetic transition, leading to a large increase in the paramagnetic regime compared to the extrapolated value from the ferromagnetic domain. Furthermore, as the Curie temperature decreases with the Cr concentration, this evolution of the diffusion properties affects the decomposition kinetics in different ways depending on the alloy composition. An atomic diffusion model, with pair interactions that depend on the local composition and on temperature, has been developed to take into account this magnetic transition effect. The interaction model has been implemented in an atomistic kinetic Monte Carlo algorithm to study the diffusion coefficients and precipitation kinetics of the Fe Cr alloys. This model has been successfully compared to decomposition kinetic experiments for a wide range of concentrations and temperatures.