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 LaCl3 and about 10 wt.-% Si. The silicon does not give X-ray powder diffraction signals. After the separation brown nanocrystalline silicon remained, with an average size of 5 nm. Conclusions about the state of the silicon in the LaCl3 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 8 K higher than the melting point of LaCl3. This marks a strong interaction between silicon and LaCl3, which is irreversibly lost after thermal treatment.