The formation of silicon powder by pyrolysis of silane diluted in argon at different concentrations has been studied. A hot wall gasphase reactor was used for the thermal decomposition of SiH4 at 1000 degreesC and atmospheric pressure. The composition, morphology, size, and shape of the particles produced has been studied utilizing electron microscopy, X-ray diffraction, infrared spectroscopy, and BET gas adsorption. Depending on the experimental conditions, agglomerates of polycrystalline, sintered particles have been obtained, which are composed of nanocrystallites of about 25 nm in size.