Several processes controlling the overall environmental fates of compounds such as the PAHs, PCBs, organochlorine pesticides, and the chlorinated dioxins depend on gas/particle partitioning in the atmosphere. These include dry deposition, wet deposition, and photolysis. Two important mechanisms by which gas/particle partitioning of SOCs to atmospheric particulate material can occur include adsorption to particle surfaces and absorption into aerosol organic matter. For aerosol materials composed solely of mineral particles (e.g., possibly continental dust or flyash from high temperature combustion), then adsorption may dominate the partitioning. For urban particulate material, however, considerable evidence is now available to indicate that absorption is the dominant partitioning mechanism. This evidence includes studies that have been carried out on clean quartz as a model mineral surface, and environmental tobacco smoke particles as a model for aerosol organic matter.