Differential scanning calorimeter (DSC) investigations and tension tests at 773 approximately 848 K have been carried out in four different superplastic Si3N4/Al-Mg-Si composites. The DSC investigations revealed that the melting point of each composite was lower than the melting point of its matrix alloy. This is probably attributed to local melting due to segregation of solute atoms near the matrix/reinforcement interfaces. The optimum superplastic temperature for each composite, where a maximum elongation was obtained, was close to its melting point. Therefore this suggested that superplastic elongations are obtained in metal matrix composites within the solid-liquid region because the presence of a liquid phase at the reinforcement/matrix interfaces serves to relieve the stress concentrations due to sliding and thereby restricts the development of internal cavitation.