Phase separation in the Fe-Cr system has been studied experimentally. The upper limit of the miscibility gap and phase separation kinetics during the early stages of decomposition was studied using Vickers micro-hardness tests and atom probe tomography. The results indicate that the upper limit of the miscibility gap in current thermodynamic descriptions is overestimated, but it is necessary to further analyze the transition across the upper limit of the miscibility gap before drawing conclusions. This work demonstrates the efficient combination of micro-hardness measurements and atom probe tomography to investigate the miscibility gap in the Fe-Cr system.