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An unstable, fc.c. Fe50Cu50 solid solution was prepared using high-energy ball milling of an elemental powder mixture. It was used as a parent alloy for isothermal annealing or low-energy ball milling (LEBM), both at 373-523 K. Alloy evolution was analyzed by X-ray diffraction, Mossbauer spectroscopy and differential scanning calorimetry. LEBM significantly enhanced the initial decomposition rate, but the decomposition process was complex and not monotonic. At and below 423 K, the initial decomposition was completely reversed at later times. The results are interpreted in terms of an effective-temperature model proposed by Martin for irradiated alloys. A dynamic phase diagram in the effective temperature is presented, and used to explain the presence of a two-phase region under all milling conditions. The nonmonotonic behavior is suggested to be a result of coupling between phase evolution and mechanical properties, leading to a time dependence of the effective temperature. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AN rights reserved.