The high-temperature plasma synthesis of ultrafine silicon nitride (Si3N4) powders through the vapour-phase reaction between SiCl4 and NH3 in an Ar/H-2 radio frequency (r.f.) inductively coupled plasma was investigated. The experiments were carried out at a 25-39 kW plate power level and at atmospheric pressure. Special attention was paid to the influence of the reactor wall temperature and plasma operating conditions on the quality of the powder. With a cold-wall reactor, the powders obtained were white to light brown in colour and were com posed of crystalline, amorphous and Si3N4 whisker phases. Both alpha and beta-Si3N4 were present in these products. The NH4Cl, formed as a by-product of the reaction, could be eliminated from the Si3N4 by thermal treatment. The BET specific surface area of the powder after thermal treatment was about 60 m(2) g(-1). The use of the hot-wall reactor resulted in a considerable reduction in the amount of NH4Cl remaining in the powder (less than 1 wt %) and a considerable increase in the fraction of the powder obtained in crystalline form. These powders were composed of a mixture of amorphous phase and 30 wt % or more of the alpha and beta-Si3N4 crystalline phases. The BET specific surface area of the powder after thermal treatment was found to be 40 m(2) g(-1). The experimental results are discussed in relation to their use for optimizing reactor design for the vapour-phase synthesis of ultrafine ceramic powders.