Spinel growth is studied by analytical electron microscopy in the fibre-matrix interface of as-processed and in thermal treated delta-Al2O3 fibre reinforced AlSi12CuMgNi alloys. In the as-processed composites a few nanometer large spinel crystals are observed in the fibre-matrix interface. They are interpreted as the reaction product of remnants of the SiO2 binder, with magnesium segregating in the interface. In the thermally treated material, large spinel crystals of a few tenths of a micrometer grow on the fibre, preferentially at positions where alpha-Al2O3 crystals reach the delta fibre surface. The spinel grows into the aluminium grain of the matrix, as well as into alpha-Al2O3 of the fibre. The orientational relationship of the spinel and the alumina is studied and the results are discussed in relation to the fibre wetting, adhesion and fibre-matrix reaction.