SiCl4/O-2 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 realtime. Si(P-3), 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 O-2 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.