The gas-phase synthesis of uranyl fluoride (UO2F2) particles via the hydrolysis of uranium hexafluoride (UF6) has been investigated in a purposely designed laboratory-scale aerosol reactor. An in situ particle sampling technique was developed for collecting particle samples from a chemically aggressive and safety critical environment. The effects of two key process variablesnamely, UF6 concentration and the molar ratio of steam to UF(6)on the particulate product properties were investigated. Image analysis of environmental scanning electron microscopy (ESEM) images of sampled particulates indicated an increase in the mean single particle size with residence time. The number and size of agglomerates also increased as a function of residence time, suggesting the presence of both surface reaction and coagulation growth mechanisms. A notable transition of the UO2F2 particles from a spherical morphology to a platelet morphology was observed when the UF6 concentration and the steam:UF6 ratio reached values of 1.3 x 10(-3) mol/L and 10, respectively.