The development of light weight alloys has received great attention in recent years; in this framework Al-Si based alloys are widely used in industries for their low thermal expansion, good resistance to wear and other properties that can be improved by the addition of rare earth elements. In the framework of a general interest in the study of the constitutional properties of light elements with rare earth metals, the isothermal sections of the Nd-Al-Si and Dy-Al-Si ternary systems at 500 °C have been investigated by using differential thermal analysis (DTA), X-ray diffraction analysis, scanning electron microscopy and electron micro-probe analysis.