The hydrology and geochemistry of groundwater in tropical island aquifers, such as on Barbados, are significantly influenced by tropical climatic conditions. Recharge to these aquifers is the product of regional and local climate patterns that control rainfall. Oxygen isotopes can be used to estimate the amount and timing of recharge on these islands because seasonal fluctuations of rainwater oxygen isotopic compositions are related to the amount of rainfall. This study shows that estimates of average annual recharge to the limestone aquifer on Barbados vary widely, displaying a more direct relationship to the distribution of rainfall throughout each year than to total annual rainfall. Recharge estimates are higher during years when rainfall is concentrated in the peak wet season months than during years when rainfall is more evenly distributed throughout the year. The El Ni¿o-Southern Oscillation appears to be partially responsible for these rainfall and recharge fluctuations. Knowledge of interannual variation of recharge and processes responsible is important because recharge variation must be considered when setting groundwater management policies related to groundwater availability.