Carbon monoxide (CO) is an important atmospheric trace gas; it is the main sink for the hydroxyl radical which oxidises many other trace gases including methane, and is also involved in the production of tropospheric ozone. Oxidation by soil microorganisms accounts for approximately one fifth of the CO sink, the remainder being removed by reaction with hydroxyl. The size of the soil sink for CO is not accurately known, and the factors which affect the rate of oxidation are not fully understood. In addition, there is evidence that soils may be net sources of CO in some circumstances. This work investigated CO fluxes between soils and the atmosphere at a variety of scales, from laboratory incubations to measurements in the nocturnal boundary layer, to identify the major controlling factors and quantify fluxes. Laboratory incubations of a wide range of Scottish soils showed that the soil total organic carbon (TOC) content was the main factor influencing CO oxidation. This may reflect increases in microbial biomass with increasing TOC content. Multiple linear regression showed that water content was also important, and further investigation showed that there was an optimum water content for oxidation which was close to the field capacity. Static chamber measurements at arable and woodland sites showed that the most important factor affecting CO oxidation within a site was temperature. In several cases there was a negative linear relationship between CO uptake rate and temperature. This unusual relationship was attributed to simultaneous CO production from dead or senescent plant litter. In contrast, fallow arable land showed an optimum rate at approximately 10 °C. During May-June 1994 continuous measurements were made of the CO concentration in clean air. This followed a diurnal pattern with decreasing during the night, increasing in the early morning, and showing little change in the late morning and afternoon. The decrease in CO concentration at night was thought to be caused by deposition to the ground, and oxidation of CO by soil microorganisms. The mean deposition velocity was 1.1 x 10 ^-3 m s ^-1, which is in reasonable agreement with values obtained by other methods. A similar experiment in July-Aug 1996 found CO deposition with a mean deposition velocity of 1.8 x 10^-3 m s ^-1.