Equilibrium Si content required to suppress the interfacial reaction in 2124 Al/SiC and A356 Al/SiC has been thermodynamically predicted by combining Wilson equation with the Miedema model. In 2124 Al/SiC composites, good agreement between thermodynamically calculated and experimental result has been observed in the solid and semi solid state reaction. At higher temperature above 750°C, thermodynamically calculated results are much higher than that of experimental values. Strong influence of reinforcement size and volume fraction on the extent of interfacial reaction is also observed in 2124 Al/SiC composites. In these composites, formation of protective layer of A1 4C3 crystals over SiC surface plays significant role in predicting the equilibrium Si content. The influence of reinforcement particle size and volume fraction on the extent of reaction has not been observed in A356 Al/SiC composites and irrespective of reaction temperature the thermodynamically calculated equilibrium Si contents for A356 Al/SiC composites agrees well with the experimental results. © 2009 TIIM, India.