Short fiber reinforced AlSi12CuMgNi composites find applications as engine components in the automobile industry. The addition of short fibers raises the interfacial reaction between the fiber and the matrix and modifies the matrix microstructure. Reaction products could change the matrix composition and alter the matrix in a physical metallurgical manner, such as the microstructure and ageing behavior. The present work investigated the microstructure in the interfacial regions and in the matrix, as well as the ageing behavior in Saffil^®- and Kaowool^®-fiber reinforced composites using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, differential thermal analysis and microhardness tests. The effects of reinforcements on the distribution of the alloying elements and on the interfacial reactions were investigated and discussed. The interfacial reaction mechanisms were analyzed based on thermodynamics and kinetics. Finally, the relationship between the interfacial reaction and ageing behavior was discussed. It was shown that the distribution of the alloying elements is affected by the addition of fibers. The degree of interfacial reaction depends on the reinforcement composition. The growth of the reaction products is controlled by the cation diffusion along the new interface between reaction product and fiber. The ageing hardening is weakened due to the depletion of the alloying element magnesium; caused by the interfacial reaction.