In metal matrix composites the adhesion between a ceramic fibre and the metal matrix should be submitted on the atomic scale to physical or chemical forces, without forming a reaction zone [1, 2]. Only in this case will fibres remain undamaged and a load transformation from the fibre to the matrix take place during composite application. For different metal matrix composites different processing methods have been developed to achieve this interface structure. In SiC-fibre reinforced titanium alloys the fibres are protected by a carbon coating [3, 4] and the mechanical properties of the composites remain unchanged as long as the protective coating prevents the reaction of titanium with the fibre surface [4]. In alumina fibre reinforced aluminium alloys fibre wetting and adhesion is submitted to atomic forces and can be supported by magnesium segregation in the interface [5, 6]. Magnesium atoms take up vacancy positions in the alumina surface, reducing the wetting angle during melt infiltration [6]. Thermal loading of these composites results in a fibre-matrix reaction that leads to a fibre and composite strength degradation [5, 6]. In carbon fibre reinforced magnesium alloys processed by hot pressing of fibre layers and matrix foils only unsufficient wetting and low adhesion between fibre and matrix result. At positions with moderate adhesion a thick magnesia layer was observed in these composites [7-9]. The magnesia formation was interpreted to arise from the oxidation of the matrix during processing, which was performed in an oxygen rich environment. This letter reports the results of transmission electron microscopy (TEM) studies on the interfaces of carbon fibre reinforced (Toray 300) magnesium alloys (AZ91:9 at% Zn, 1 at% A1, Mn, Si and Cu impurities) processed by squeeze casting of chopped carbon fibre preform bonded by an SiO2 binder (4 wt %). The fibre preform was heated to 600 °C before melt infiltration, which took place at 812 °C. Composites with a fibre volume fraction of 20% were obtained [10]. In the preform the fibres have a preferential orientation in a plane perpendicular to the infiltration direction. For TEM investigations sample preparation was performed parallel and perpendicular to the preferential fibre orientation plane. Preparation was performed in the usual way by grinding,