Mercury is emitted to the air from Hg-enriched and low Hg-containing (natural background) substrates. Emitted Hg can be geogenic, or can be derived from the re-emission of Hg that was previously deposited to the soil from the atmosphere. Atmospheric Hg can be derived from natural and/or anthropogenic sources and can be deposited by wet or dry processes. It is important to understand the relative magnitude of emission, deposition, and re-emission of Hg associated with terrestrial ecosystems with natural background soil Hg concentrations because these landscapes cover large terrestrial surface areas. This information is also important for developing biogeochemical mass balances, assessing the impacts of atmospheric Hg sources, and predicting the effectiveness of regulatory controls at local, regional, and global scales. The major focus of this paper is to discuss air–substrate Hg exchange for low Hg-containing soils (<0.1 μg Hg g−1) from two areas in Nevada and one in Oklahoma, USA. Data collected with field and laboratory gas exchange systems are presented. Results indicate that in order to adequately characterize substrate–air Hg exchange, diel and seasonal data must be collected under a variety of environmental conditions. Field and laboratory data showed that dry deposition of gaseous Hg to substrates with low Hg concentrations is an important process. Environmental parameters important in influencing emissions include soil water content, incident light, temperature, atmospheric oxidants, and air Hg concentrations. There are synergistic and antagonistic effects between these parameters complicating prediction of flux.