This work investigates the alloying reaction undergone by Al50Fe50 powder mixtures submitted to mechanical processing by ball milling. The transformation kinetics was studied by quantitative X-ray diffraction. Experimental evidences indicate that Al gradually dissolves in Fe, finally forming a crystalline solid solution. A phenomenological model was developed to describe the observed kinetics with reference to the number of collisions and to the fraction of powder effectively processed at individual collisions. It is shown that only about 5 mu g of powders are involved in the Al dissolution processes at collision. It is also shown that a Al30Fe70 solid solution already forms at the first impact via local dissolution processes. (C) 2012 Elsevier B.V. All rights reserved.