A source of silver iodide aerosols has been studied to determine the effect of the temperature of generation on the crystal structure of the particles in the aerosol output. Sampling was done by use of a novel thermal precipitator which gave samples in the proper form for direct analysis in a Debye‐Scherrer x‐ray diffraction camera. Patterns were obtained indicating the presence of both cubic and hexagonal modifications (ZnS and ZnO type). As the source temperature was raised, an increase of the proportion of hexagonal particles was observed, 95 percent hexagonal particles resulted when the source was operated at 800°C. An explanation of this behavior is proposed.