Phase separation in a commercial Fe-20 wt.%Cr-6%Al oxide dispersion-strengthened PM 2000 steel has been characterized with a local-electrode atom probe after isothermal aging at 708 K and 748 K for times up to 3,600 h. A progressing decrease in the Al content of the Cr-rich alpha' phase was observed with time at both aging temperatures. The Al partitioning trend was consistent with theoretical calculations. However, the experimentally observed Al partitioning factor was significantly lower than the predicted equilibrium value. A similar to 10 nm diameter, roughly spherical, Al- and Ti-enriched beta' Fe(AlTi) phase was also observed.