This work investigates the decomposition of tetramethylsilane and the formation of silicon oxide clusters in a laminar premixed low-pressure hydrogen flame using molecular-beam mass spectrometry (MBMS). A comprehensive list of the species that exist in the gas phase was compiled and spatially resolved mole fraction profiles of species in the flame were obtained. Quantitative data in dependence of height above the burner were obtained for all major species and intermediates. The MBMS detection technique allowed to monitor Si-C-O-H, and Si-O-H-containing compounds as well as C-1-C-2 species. The measured data show that the reaction of tetramethylsilane is initiated by H-abstraction from a methyl group and subsequent formation of oxygenated species. The measurements suggest that combustion of tetramethylsilane in a hydrogen flame proceeds mainly by a stepwise substitution of the methyl ligands by hydroxyl groups. Molecular and radical intermediates like Si(CH3)(2)OH, Si(OH)(3), and Si(OH)(4) are formed in the reaction zone. Significant amounts of Si(OH)(4) are present at large distances above the burner. A repetitive growth pattern suggests that the monomer Si(OH)(4) is a likely species initiating the formation and growth of larger silicon oxide clusters, e.g., Si4O10H4,Si5O12H4, and Si6O14H4, that can form nanoparticles in subsequent reactions.