We have collected and analyzed air samples for 13CO, C18O, and CO concentration at Barbados, West Indies (13¿N, 59¿W), from July 1996 to May 2000. The δ13C values during the summer of 1996 and 1997 are among the lowest values reported for atmospheric CO, at less than -34? (Peedee belemnite-CO2), but seasonal maxima are close to that reported at other stations. Seasonal lows for 1998 and 1999 are between -32.5? and -33?, which is also lower than observations at higher-latitude sites. Such low values are difficult to simulate by considering the well-established CO sources alone, and may indicate a small, but very isotopically depleted, source of CO from the marine boundary layer, such as chlorine-catalyzed destruction of CH4. We directly compare the observations at Barbados with data from middle-latitude and high-latitude Northern Hemisphere sites and construct a latitudinal dependence in the seasonal amplitude of both 13CO and C18O. We then investigate the source strengths of CO at this site using a three-dimensional inverse model analysis, which has also been used to simulate the other data sets. Model simulations reproduce the seasonal variations in , δ13C, and δ18O but do not predict the large amplitude of δ13C (which is due to the very depleted summertime δ13C values) or the absolute values for 18O. We also include a calculation of the relative strengths of certain sources based on the model inversion.