The temporal evolution of the microstructure resulting from phase separation into Fe-rich (α), Cr-rich (α¢), and Fe(Ti,Al) (β¢) phases of a Fe-20Cr-6Al-0.5Ti alloy has been analyzed by thermoelectric power measurements (TEP). The early stages of decomposition and the evolution of the three-dimensional microstructure have been analyzed by atom probe tomography (APT). The roles of Cr, Al, and Ti during the decomposition process have been investigated in terms of solute partitioning between the phases. Analysis of proximity histograms revealed that significant Al and Ti partitioning occurs, which is consistent with theoretical calculations. The results indicate that as the α-α¢ phase separation proceeds, Al and Ti are rejected into the α phase, which causes the β¢ phase to nucleate on the surface of the α¢ phase.