The solid state reaction between Al and Fe3O4 (magnetite) using mechanochemical activation of powder mixtures under Ar atmosphere is studied. The phase evolution during the reaction is analyzed by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). At 37 minutes of high-energy ball-milling the disappearance of reactive phases and the production of alpha-Fe, FeAl2O4 and alpha-Al2O3 is observed, together with significant changes in the magnetic behavior of the system. The composition and properties of samples heated up to 1200degreesC are also investigated. The behavior of the saturation magnetization M-s is interpreted on the basis of the formation of a variable composition spinel phase Fe [Al-x Fe2-x] O-4 with 0 less than or equal to x less than or equal to 2 and a canting effect due to the presence of Al3+ ions in the spinel structure.