The formation of zirconia particles prepared by low-pressure spray pyrolysis (LPSP) was studied experimentally and numerically. A numerical model was developed, and it is the first to quantitatively explain the phenomena occurring in the LPSP system. Under certain operational conditions, particle formation was based on a one-droplet to one-particle conversion, as found in the spray pyrolysis process under atmospheric pressure conditions. On the other hand, nanoparticles were generated at higher temperatures and under conditions of lower pressure. Zirconia nanoparticles were successfully produced under experimental conditions of 1600 degrees C, 30 Torr with 10 l/min nitrogen as a carrier gas. The mass and heat transport calculation indicated that the evaporation rate would increase rapidly with increasing temperature and decreasing pressure in the furnace. The model considered flow regimes, and accounted for the regime range from continuum to free-molecular, depending on the Knudsen number. A very high evaporation rate led to the assumption that droplets were ruptured instantaneously, resulting in the production of monomers. Nanoparticles were then formed by the nucleation of monomers to form clusters, and then by the coagulation between clusters and surface condensation of monomers onto clusters. To predict the size distribution of nanoparticles, a population balance analysis called the nodal method was used. The simulation results showed reasonable agreement with the experimental results. (C) 2009 Elsevier Ltd. All rights reserved.