Thermal plasma treatment of ceramic particles of different shape makes them spherical. The spherical particles are hollow or porous under certain experimental conditions. The objective of the present work was to investigate how the water content and porosity of raw materials and the composition of the gas phase influence the size of the hollows. We concluded that the porous structure is the major factor contributing to the formation of hollow spheres, while high initial water content is not a prerequisite for the formation of internal cavities. (C) 2003 Elsevier Science B.V. All rights reserved.