Silicon nanoparticles have been prepared by many methods, depending on whether porous, amorphous, or crystalline silicon is required. The advantages of the novel solid-gas chemical reaction reported here are that large amounts can be produced in a convenient way, the particles have a very narrow size range, and there is enormous potential for modification of the size and surface of the particles. Details of the synthesis via reaction of lanthanum or LaCl3 with a gaseous mixture of silicon tetrachloride and hydrogen, together with the subsequent characterization of the product, are presented.