Binary iron alloys containing 6, 10, 15 and 20 at.% Mo were investigated. It was found that the "ordering-phase separation" transition is a time depending process: at the beginning of isothermal aging at 550 degrees C, a modulated structure is formed in the solid solution as a result of the tendency toward phase separation; with time, Fe2Mo particles precipitate in the modulated structure dissolving as a result of the tendency toward ordering. Further aging at 550 degrees C leads to the complete dissolution of the modulated structure and Fe2Mo particles remain the only phase precipitated in the solid solution. (C) 2010 Elsevier B.V. All rights reserved.