The BOREAS group TE-05 collected measurements in the NSA and SSA on gas exchange, gas composition and tree growth. This data set contains measurements of the concentration and stable carbon (13C/12C) and oxygen (18O/16O) isotope ratios of atmospheric CO2 in air samples collected at different heights within forest canopies. The data were collected to determine the influence of photosynthesis and respiration by the forest ecosystems on the concentration and stable isotope ratio of atmospheric CO2. These measurements were collected at the SSA during each IFC at OJP, OBS, and OA sites. Measurements were also collected at the NSA during each 1994 IFC at the OJP, T6R5S TE UBS, and T2Q6A TE OA sites. The stable isotope ratios are expressed using standard delta notation and in units of per mil. The isotope ratios are expressed relative to the international standard, PDB, for both carbon and oxygen samples.