An investigation has been made of composites with magnesium-based matrices CPMg, AZ61 and AZ91 reinforced with Safimax low-density (LD), standard-density (SD) and RF Saffil alumina fibres, using either a squeeze or a gas-pressure casting route. Detailed investigations of structural features have been made using SEM, TEM and EDX analysis. The overall extent of reaction between matrix and fibre was affected by the volume fraction of fibres and (locally) by the formation of metal channels between fibre bundles. Fibre microstructure and porosity are the key features which significantly influence the extent of chemical interaction. LD (Safimax) alumina fibres were fully reacted and cannot be employed to produce liquid-metal infiltrated composites, unless a method to stabilize or protect the fibres can be found. In the case of SD Safimax fibres, the metal/ceramic interaction produced a considerable penetration of magnesium into the fibres. However, there was negligible chemical reaction in composites containing RF Saffil alumina fibres.