An aerosol reactor is a gaseous system in which fine particles are formed by chemical reaction in either a batch or flow process. Such reactors are used to study the aerosol formation process, as in a smog reactor, or to generate a product such as a pigment or a catalytic aerosol. This paper deals with reactors in which aerosol material is generated at a constant rate. Design parameters of interest are the size distribution, number density, surface area, and mass loadings. For ideal systems composed of spherical coalescing particles, these can be predicted theoretically for certain limiting cases. However, the irregular agglomerates which may form are more difficult to characterize theoretically.