A model for aerosol-phase densification of particles during their synthesis by spray pyrolysis is presented. The model was used to describe the evolution of the specific surface area of PdO powders synthesized at temperatures between 400 to 800 °C (residence times 27.9 to 22.7 s). Surface areas and grain sizes ranged between 56 m2/g and 4 nm at 400 °C to 3.2 m2/g and 40 nm at 800 °C The characteristic coalescence lime was determined as: τ= 1.7 x 1013T d3exp(1.3 x 105/RT) [for lattice diffusion] and t = 2.7 x 1019Td4exp (1.6 x 105/RT) [for grain boundary diffusion] (τ is in seconds,Tin degrees Kelvin, R is 8.314J/(mol.K)and d is in meters), but the data and model did not allow identification of the specific mechanism. The model provides a general approach for correlating changes in particle surface area with reactor operating conditions that is applicable to a wide variety of materials.