The microstructure of different Fe-Cr-Al alloys in the composition range of 0 to 18 at % Cr and 7 to 25 at % Al was investigated by transmission electron microscopy (TEM). By systematically varying the compositions, the location of the order-disorder transition was determined. Dark field images confirmed that many samples consisted of a two-phase mixture of small DO3-ordered particles in a disordered A2 matrix. The mottled contrast exhibited by some samples and sometimes taken as a proof of the existence of ordered particles is shown to be produced by an artefact due to the TEM sample preparation.