Soil atmosphere fluxes of carbon monoxide (CO) were investigated during BOREAS 1994 (June to September 1994) in forest sites near the northern study area (NSA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). Fluxes and related ancillary data were measured for both upland black spruce (located on poorly drained clay-textured soils) and jack pine sites (well-drained sandy soils) that were in early stages of succession following stand replacement fires that occurred within 7 years of BOREAS 1994. Nearby control stands that had not burned in the past 80 years were studied for comparison. Net fluxes measured by using transparent closed chambers were generally positive at the warmer, sunlit burn sites but negative (sink activity) in the shaded, cooler control sites. Carbon monoxide uptake in controls, which was first order with respect to CO concentration, was little affected by covering the sampling chambers to exclude light. Median deposition velocities calculated from the uptake fluxes were 0.015 cm s-1 at the black spruce control site and 0.0085 cm s-1 at the jack pine control sites, at the lower end of the range of values observed by others in tropical and temperate ecosystems. Daytime CO fluxes at the burn sites were generally positive (1011-1012 molecules cm-2s-1) and were lowered when solar irradiance was excluded from the chambers by covering or when cloudiness or smoke reduced the light intensity. Net fluxes at the burn sites were controlled by competition between abiotic production, mainly at the surface, and by oxidation deeper in the soil. Abiotic production, which was attributable to photoproduction and thermal decomposition of the surface organic layer and charcoal, strongly correlated with incident solar irradiance, and thus the greatest fluxes were observed during midday. Results of these studies indicate that the locally dependent changes in boreal fire return intervals that are linked to global climate change represent an important biospheric/physical feedback that is likely to alter the biosphere-atmosphere exchange of CO.