Production and deposition rates of atmospheric hydrogen and carbon monoxide were studied during field measurements in subtropical regions, i.e., Transvaal (South Africa), Andalusia (Spain), and the Karoo (South Africa). Measurements were carried out by applying static and equilibrium box techniques. The equilibrium technique has been introduced as a novel method to measure production and destruction rates simultaneously even when soil conditions (e.g., temperature) change during the course of the measurements. Deposition velocities of H2 and CO were virtually independent of the soil temperature measured in 3- to 10-mm depths and agreed with those measured in the temperate regions. The deposition velocities were inhibited or stimulated by irrigation water depending on the conditions of the individual field sites. H2 production by soil was not observed. By contrast, CO was produced by soil in a dark chemical reaction. Production rates increased exponentially with soil temperatures, giving activation energies of 57--110 kJ mol-1 and increased linearly with soil organic carbon content. CO production rates followed a diel rhythm parallel to soil surface temperatures. Production generally exceeded CO deposition during the hot hours of the day, so that arid subtropical soils act as a net source of atmospheric CO during this time. On a global basis, CO production by soil may reach source strengths of 30 Tg yr-1, which is considerably less than the global deposition of CO estimated to be 190--580 Tg yr-1. Global H2 deposition rates were estimated to 70--110 Tg yr-1.