An atomic scale analysis of phase separation in a thermally aged Fe–25at.% Cr alloy at 500°C using 3-D atom probe (3DAP) and atomistic kinetic Monte Carlo (AKMC) simulation is presented. Treatment of the simulation data with the Lifshitz–Slyozov–Wagner and Huse laws shows that, whereas diffusion along the interfaces and through the bulk both occur at early stages, diffusion through the matrix quickly controls the growth of domains whereas the structure is still interconnected. Comparison of AKMC results with experimental ones showed that AKMC simulation in the two-band model approximation on a rigid lattice is able to reproduce the behaviour of the concentration field and of the width of the domains observed with 3DAP. This comparison also strongly support that, in real Fe–Cr alloys, as well as in simulated systems, diffusion is predominantly through the bulk and controls the growth of domains, while the structure is still interconnected.