The introduction of alloying elements in an aluminium matrix may have a detrimental effect on the mechanical properties of unidirectional composites. The aim of this work was to establish a correlation between the mechanical properties of Aluminium based/Altex fibers MMCs processed by medium pressure infiltration and their microstructure, the composition of the matrix and the mechanical properties of the fibers. Al-Mg, Al-Si, Al-Mg-Si and Al-Mg-Si-Cu matrices were studied. The microstructure of the composites was characterized by optical and electron microscopy observations. Transmission electron microscopy was also used to investigate the reactivity phenomena likely to occur between the matrix and the fibers. Furthermore, the tensile properties of the fibers after introduction in the metal matrix were measured by single fiber tests, to assess the degradation of the fibers. The tensile properties of the composites were also determined both in longitudinal and transverse direction. They were interpreted referring to the microstructural state of the composites and to the tensile properties of the fibers. Lastly, the detrimental effects due to the alloying elements (solid or liquid state reactivity, presence of brittle phases, matrix hardening) were classified according to their importance.