Due to the foreseen possibility of technological applications, there is an increasing interest in nanocrystalline soft materials, especially in iron silicon alloys. The synthesis of nanoparticles by means of CO2 laser induced pyrolisys is an important method for the production of the latter material with high purity upon well controlled conditions. In this production process, implying fast ultrafine powders condensation from the gas phase, the temperature is a key parameter, since the particles size is strongly affected by its variations. The potentialities of laser spectroscopic methods in the ultraviolet and infrared spectral region to monitor nanoparticles growth in a semi-industrial flow reactor for production of Si and FeSi nanoparticles and operating in ENEA, are explored in the present work.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.