We describe a process in which nanosize particles with a narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles ave characterized by enthalpy probe measurements, by calorimetric energy balances, and by a model of the nozzle flow Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nm. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 nm or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initialed by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reaction sat the particle surfaces involving carbon-containing species from the gas phase.