In the present work, we have produced aluminium matrix composites reinforced with a ceramic pre-form of saffil fibres, by using a low-pressure infiltration method. Then, the aim of this work is to characterise the reaction, between the saffil delta-alumina fibres of the pre-form and the aluminium, that takes place during the infiltration process of the liquid aluminium and the further slow cooling down of the infiltrated composite. The use of scanning and transmission electron microscopy, X-ray diffraction and microanalysis has allowed us to describe and identify the reaction compounds. The results show that the main reaction of the aluminium takes place with the silicon oxide film of the binder covering the fibres, given place to the superficial crystallisation of alpha-alumina crystals, as well as some cubic crystals.