SiCl4/O2 mixtures were photolyzed by focused ArF excimer laser pulses, where oxide particles were produced at room temperature (296 ± 3 K). Measurements of laser-induced fluorescence (LIF) as well as light scattering by particles were conducted in the reaction zone in order to observe the evolution of transient molecules in real time. Si(3P), SiCl, and SiCl2 were observed as direct photoproducts, and SiO was observed as a product of the initial oxidation process. Si and SiCl were confirmed to react with O2 rapidly, whereas SiCl2 was totally inert at room temperatures and reduced pressures (15-60 Torr in Ar). SiO was found to have second-order decay rates to the concentration of SiO itself. This may be attributed to the recombination of SiO forming a dimer. By comparing the evolution of SiO with the particle growth rate deduced from the scattering signals, the reaction mechanism for particle formation is discussed.