Hollow ceramic microspheres are facing continuously growing applications as was described by Cochran [1]. Thus, it is not surprising at all that research on them has been extended more recently. Various processing techniques for making ceramic hollow spheres of 5–200 micron diameter have been developed. One promising method is based on sol-gel processing [2–5]. Particular method involves the dispersion of an aqueous colloidal sol of metal oxide in a dehydrating liquid which extracts water from the interior of sol droplets. A second method comprises the coating of a pre-processed polymer or glass microsphere with ceramic gel followed by firing [6]. In other methods, atomized liquid droplets or spray dried powders passing through a furnace or a thermal plasma flame give rise to hollow microspheres [7–9]. Thermal plasma ensures adequately high temperature for melting the ceramic particles. Water vapor or other gases liberated due to the decomposition of binder or blowing agent, which is usually blended to the agglomerates, are supposed to form internal cavity. By controlling the operating conditions, hollow particles can be formed even during the spray drying process [10, 11]. The present report describes the preparation of hollow silica microspheres by inductively coupled RF thermal plasma. The research was initiated and supported by PREMIS Technologies, France. The effect of process gases and the type of the starting materials on the microstructure of the formed particles was studied in the experiments. The experimental apparatus consisted of a TEKNAtype induction plasma torch (PL-035LS) with a quartz confinement tube of 25 mm and a water cooled steel chamber connected to a cyclon. The plasma plate power of 21 kW was provided by a three turn, water cooled induction coil from an RF generator operating at an oscillator frequency of 3 MHz. High purity argon was used both as plasma and sheath gas with flow rates of 20 and 60 l min−1, respectively. Hydrogen was mixed into the sheath gas with a proportion of 10 V/V% to improve the enthalpy and the heat transfer coefficient of argon. In some experiments, air was substituted for argon. The powders were fed with a PRAXAIR powder feeder unit through an injection probe to the top of the plasma flame by argon carrier gas (3 l min−1) with a constant feed rate of 10 g min−1. The outlet of the quartz injection tube has an inner diameter of 2 mm. It was located 10 mm below the top of the induction coil. Three different raw materials were investigated in the experiments. Powder Type 1 of negligible porosity (from Degussa) is made by precipitation of pure silica. Powder Type 2 is an agglomerated sample, in which the diameter of primary silica particles (from SIFRACO) is ∼5 μm. Besides silica, NaNO3 and Na-silicates were also blended into Powder Type 2 for different purposes. Na-silicates were used as binding agent of primary particles, while the blowing agent, NaNO3 was devoted to release different gases (O2, N2) at higher temperatures by decomposition. Powder Type 3 is also an agglomerated silica sample (from SIFRACO). It differs from Powder Type 2 in that it contains only polyvinyl alcohol (PVA) as binder about 1% by weight. Both the raw materials and the produced powders were characterized for density, specific surface area, phase composition (XRD) and microstructure (SEM). Density was determined by means of a helium picnometer (Micromeritics AccuPyc 1300). The specific surface areas were measured with a conventional volumetric instrument by BET method. The microstructural (SEM) investigations were performed on a Jeol JSN50A apparatus. Cross-section images of the particles were made by embedding them in resin media followed by their cutting with diamond blade. During the in-flight process of plasma spraying silica ceramic grains are melted. As a result, one anticipates formation of solid particles of reduced size. It is reasoned by the better space filling of melts as compared to solids. In addition particles can also evaporate to some extent. Spheroidization by plasma flame takes the advantage of this phenomenon [12, 13]. Particles, however, can also become hollow. Formation of hollow particles can be attributed to the release of gases in the interior of particles, if they cannot break through the outer melted layer. The freezing temperature of the