CO fluxes between soil and atmosphere were measured by field studies under arid conditions in Germany as well as in a semidesert (Karoo) and a desert (Namib) in South and Southwest Africa. Arid soils usually acted as a net source of atmospheric CO. The CO emission rates were dependent on soil temperature and soil organic carbon content. CO emission turned into CO deposition, when the soils were irrigated. CO deposition was due to microbial CO consumption, whereas CO emission was due to chemical processes.