Persistent organic chemicals (POCs) undergo processes similar to the biogeochemical cycling of elements such as carbon. However, while kinetics of transformation control the cycling of carbon, the processing of POCs is mainly dictated by the thermodynamics of equilibrium partitioning. Fugacity, the “escaping tendency” of a chemical from one medium to another, plays a key role. Forests have been shown to be globally important compartments in scavenging airborne POCs and translocating them to the upper soil layer in falling litter, i.e. they are significant sinks of POCs. However, it is unknown how litter degradation affects the POC fugacity. The overall goal of the proposed research project is to investigate thermodynamic controls that determine the cycling of POCs between air, vegetation and soils in boreal, temperate and subtropical forests.