On July 15 and 16, 1996, profiles of temperature, water vapor, carbon dioxide concentration, and its carbon isotopic composition were made within and above the convective boundary layer (CBL), near the village of Zotino in central Siberia (60¿N, 89¿E). On both days the CBL grew to a height of around 1000 m at midday after which little further growth was observed. This was despite high rates of sensible heat flux into the CBL from the predominantly coniferous vegetation below and was attributable to a high subsidence velocity. For all flights, marked discontinuities across the top of the CBL were observed for water vapor and CO2 concentrations with differences between the CBL and the free troposphere above being as high as 10 mmol mol-1 and 13 &mgr;mol mol-1, respectively. Associated with the lower CO2 concentrations within the CBL was an enrichment of the &dgr; 13C in CO2 of up to 0.7?. Although for any one flight, fluctuations in CO2 and &dgr; 13C within the CBL were small (less than 3 &mgr;mol mol-1 and 0.1?); they were well correlated and suggested a photosynthetic discrimination, &Dgr;, by the vegetation below of ~17?. Estimates of regional &Dgr; based on CBL budgeting techniques suggested values ranging from 14.8 to 20.4?. CBL budgeting techniques were also used to estimate regional ecosystem carbon fluxes (-3 to -9 &mgr;mol m-2 s-1) and evaporation rates (1--3 mmol m-2 s-1). Agreement with ground-based tower measurements was reasonable, but a bootstrap error analysis suggested that errors associated with the integral CBL technique were sometimes unacceptably large, especially for estimates of regional photosynthetic 13C discrimination and regional evaporation rates. Conditions under which CBL techniques should result in reasonably accurate estimations of regional fluxes and isotopic fractionations are evaluated. ¿ 2001 American Geophysical Union