The present invention relates to a process for the synthesis of a composite material comprising steps of: (a) reacting gaseous magnesium (Mg) and silica (SiO2) in an inert atmosphere; (b) washing the product obtained in step (a) in an acidic medium; and (c) reacting further gaseous magnesium (Mg) with the silica (SiO2) and silicon (Si) product obtained in step (b). The process of the invention allows Mg2Si/MgO nanocomposites to be prepared without too many separate steps, and wherein the MgO phase is homogeneously dispersed within the Mg2Si matrix. The nanocomposites obtained may for example find practical application as thermoelectric materials in thermoelectric generators.