Our objective was to analyze factors that influence changes in the oxygen isotope ratio (&dgr;18O) of atmospheric CO2 within boreal forest ecosystems. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem-Atmosphere Study (BOREAS). In terrestrial ecosystems the &dgr;18O value of atmospheric CO2is strongly influence by isotope effects that occur during photosynthesis and respiration. Of primary importance is an equilibrium isotope effect that occurs between oxygen in CO2 and oxygen in soil water and plant chloroplast water. During the equilibrium reaction the oxygen isotope ratio of CO2 becomes enriched in 18O relative to that of water. We measured seasonal changes in the oxygen isotope ratio of (1) water input to the ecosystems (precipitation), (2) water taken up by the major plant species from the soil (plant stem water), and (3) water in plant leaves. We used this information in calculations of isotope discrimination during photosynthesis and soil respiration. Discrimination against C18O16O during photosynthetic gas exchange (&Dgr;A (influenced by equilibration with chloroplast water) averaged approximately 21? at midday and was similar for all forest types. In contrast, CO2 released during plant and soil respiration had an average &dgr;18O value of -14.4? but was less depleted in 18O than would be expected for respired CO2 in isotopic equilibrium with soil water. This effect was most pronounced in black spruce sites because of the extensive coverage of moss on the ground surface and the observation that water in the upper moss layers can have an oxygen isotope ratio substantially different from water in deepersoil layers.¿ 1997 American Geophysical Union