As the extent of the interfacial reactions controls the properties of metal matrix composites, the microstructural features and the chemical composition of the interlayers in aluminium wires reinforced with unidirectional carbon fibres (volume fraction app. 55%) have been investigated. High voltage and high resolution transmission electron microscopy of fibres, matrix, and interlayers, combined with analytical methods (electron energy loss spectroscopy and energy filtered microscopy) revealed a nanometre-sized C/Al interdiffusion layer and aluminium carbide needles or platelets of 10-50 nm thickness and 50-500 nm length in the matrix material, starting from the interlayer, the extension of which strongly correlates with the duration of melt contact. The observed interlayer phenomena impose restrictions to the process parameters, as by massive interface reactions the fibre strength is degraded, and the formation of brittle reaction products such as Al4C3 provides sites for initiation of fibre cracking and can cause composite failure. With a newly developed continuous process, which is capable of infiltrating endless products, the fibre/melt contact duration could be reduced to less than one second resulting in carbide formation lower than 0.2 wt% as confirmed by chemical analyses. So it was possible to achieve strength values of the composite wires that are as high as the theoretical prediction.