The heterogeneous reaction of lanthanum or lanthanum(III) chloride with a gas mixture of hydrogen and silicon tetrachloride is a novel synthesis route for silicon nanocrystals. The reaction product consists of LaCl 3 and about 10wt.-% Si. The silicon does not give X-ray powder diffraction signals. After the separation brown nanocrystalline silicon remained, with an average size of 5nm. Conclusions about the state of the silicon in the LaCl 3 matrix are drawn by the thermal characterization of the reaction product. The thermal analysis curve of the reaction product shows, at the first cycle, an endothermic effect, which is about 8K higher than the melting point of LaCl 3 . This marks a strong interaction between silicon and LaCl 3 , which is irreversibly lost after thermal treatment.