Profiles of SiO mole fraction have been obtained in low pressure premixed hydrogen/oxygen/hexamethyldisilazane (HMDS) flames by laser-induced fluorescence and emission spectroscopy. SiO is observed to form early in the flame at temperatures near 1100 K. The flame equivalence ratio has only a small effect on the SiO mole fraction, but the precursor concentration has a marked effect on both the peak mole fraction and shape of the profile. The SiO observed in the flame appears to be from two sources: SiO produced directly from precursor pyrolysis, and SiO that is generated as the partial pressure from the solid SiO in the flame. A comparison of ground state and excited state profiles indicates that the emission from SiO is chemiluminescence rather than thermal emission. A computational flow model incorporating current Si/H/O chemical mechanisms is able to qualitatively reproduce some aspects of the observed profiles, but significant discrepancies remain. The profiles presented in this study can serve as a test against which future chemical and process models may be validated. (C) 2001 by The Combustion Institute.