Investigations have been carried out to determine the conditions that lead to the production of spherical hollow ceramic oxide particles during melting in a d.c. plasma jet. Reports in the literature indicated that such ceramic particles were formed by plasma spraying spray dried agglomerates, but precise details of the conditions necessary for their formation were not stated. In this study it is shown that for hollow particles to be formed several conditions had to be met. Spherical spray dried agglomerates had to be used as starting materials, the material being sprayed had to melt over a narrow temperature range and the size of the particles had to exceed a certain diameter. Experiments, using yttria, showed that the relative size of the pore was dependent on particle diameter, and it has been proposed that the major controlling factor that influences this dependence is the escape of gas trapped in the spray dried agglomerate during melting rather than surface tension or undercooling which were shown to produce only minor effects. In addition, the results also showed that the nature of porosity within the hollow particles as well as the surface morphology was dependent on the material being sprayed.