This paper summarizes some features of the interannual variability of tropical CO2 sources during 1980-95. Sources are derived from inversion of atmospheric concentration and isotopic data using three different techniques and two different transport models. We show that the tropical source is significantly correlated with the SOI. Composite CO2 sources for ENSO events show an initial negative anomaly followed by a positive anomaly. We tentatively attribute the negative anomaly to the ocean and the positive anomaly to a terrestrial response.