In situ Fourier transform infrared absorption spectroscopy has been used to study the composition of particles formed and suspended in radio-frequency discharges of silane-oxygen-argon gas mixtures. The silane gas constumption was observed by infrared absorption. The stoichiometry of the produced particles depends on the silane flow rate and was compared with commercial colloidal silica. A small proportion of silane gas produces nanometric stoichiometric particles whereas a large proportion produces larger under-stoichiometric particles. Absorption spectroscopy was sufficiently sensitive to reveal particles too small to be visually observed by laser light scattering. Post-oxidation of hydrogenated silicon particles trapped in an argon plasma was obtained by adding oxygen. Mass spectrometry of negative and positive ions showed an extensive range of ionic clusters which may be at the origin of the observed particle formation. A model based on an iterative reaction sequence gives a good agreement with the measured positive ion mass spectrum.