Records of atmospheric CO2 and 13CO2 can be used to distinguish terrestrial vs. oceanic exchanges of CO2 with the atmosphere. However, this approach has proven difficult in the tropics, partly due to extensive land conversion from C3 to C4 vegetation. We estimated the effects of such conversion on biosphere-atmosphere 13C exchange for 1991--2000, and then explored how this land-use disequilibrium altered the partitioning of net atmospheric CO2 exchanges between ocean and land using NOAA-CMDL data and a 2D, zonally averaged atmospheric transport model. Our results suggest sizable CO2 uptake in C3-dominated tropical regions in 8 of the 10 years; 1997 and 1998, which included a strong ENSO event, are near neutral. Since these fluxes include any deforestation source, our findings imply either that such sources are smaller than previously estimated, and/or the existence of a large equatorial terrestrial CO2 sink.