The microstructure and mechanical properties of two aluminium-based composites reinforced with Nicalon fibre are investigated. During composite processing, aluminium carbide forms at the interface as a result of a reaction between aluminium and free Magnesium, when present in the aluminium matrix, diffuses into the outer (approximately 200 nm) layer of the fibre where it reacts with the silicon oxycarbide constituent to form magnesium-containing oxide and also to free carbon for the production of more interfacial aluminium carbide. These chemical reactions affect to differing degrees the strength of a fibre, as measured after extraction from the two composites, and influence the respective fibre/matrix interfacial friction stress and composite strength. A simple rule-of-mixtures approach based upon the measured strength of extracted fibres gave some agreement with longitudinal properties of the composite, but treatment of the fibres as bundles, using a Weibull probability distribution of properties, provided more accurate predictions.