In this study, using a three-dimensional (3-D) tracer modeling approach, we simulate the Δ18O of atmospheric CO2. In the atmospheric transport model TM2 we prescribe the surface fluxes of 18O due to vegetation and soils, ocean exchange, fossil emissions, and biomass burning. The model simulations are first discussed for each reservoir separately, then all the reservoirs are combined to allow a comparison with the atmospheric Δ18O measurements made by the National Oceanic and Atmospheric Administration-University of Colorado, Scripps Institution of Oceanography-Centrum Voor Isotopen Onderzoek (United States-Netherlands) and Commonwealth Scientific and Industrial Research Organisation (Australia) air sampling programs. Insights into the latitudinal differences and into the seasonal cycle of Δ18O in CO2 are gained by looking at the contribution of each source. The isotopic exchange with soils induces a large isotopic depletion over the northern hemisphere continents, which overcomes the concurrent effect of isotopic enrichment due to leaf exchange. Compared to the land biota, the ocean fluxes and the anthropogenic CO2 source have a relatively minor influence. The shape of the latitudinal profile in Δ18O appears determined primarily by the respiration of the land biota, which balances photosynthetic uptake over the course of a year. Additional information on the phasing of the terrestrial carbon exchange comes from the seasonal cycle of Δ18O at high northern latitudes.¿ 1997 American Geophysical Union