During silicon thermal chemical vapor deposition, reactions occurring in the gas phase above the wafer surface can strongly influence the deposited film quality. Depending on process conditions (e.g., temperature, silicon precursor, carrier gas, pressure) gas phase reactions can include not only precursor decomposition but also nucleation of silicon nano‐particles above the wafer surface. Optical diagnostics were employed to investigate such processes during silicon chemical vapor deposition via silane pyrolysis. Measurements were performed in a vertical flow, rotating disk reactor under various process conditions. Gas phase silicon particle spatial distributions were determined with elastic light scattering. Chemical composition of the particles was investigatedin situwith vibrational Raman spectroscopy. Raman spectra showed that the particles were composed of amorphous silicon and/or crystalline silicon, depending on growth temperature. Raman spectral features of the crystalline silicon also indicated crystalline domain sizes in the ca. 3 nm to ca. 10 nm size range. Gas phase temperature measurements (in the absence of particles) were used to estimate an amorphous‐to‐crystalline silicon transition temperature of ca. 866 K. © 2003 American Institute of Physics