We use two atmospheric general circulation models (AGCMs), the ECHAM-4 and the GISS II models, to analyze the interannual variability of δ18O in precipitation over the tropical Americas. Several different simulations with isotopic tracers forced with observed global sea surface temperatures (SST) between 1950 and 1998 reveal the influence of varying temperature, precipitation amount, and moisture source contributions on the predicted δ18O distribution. Observational evidence from climatic (NCEP-NCAR) and sparse stable isotope (IAEA-GNIP) data is used to evaluate model performance. The models capture the essential features of surface climate over the tropical Americas in terms of both their spatial and temporal characteristics. Using a low-resolution model (GISS II), adjusted to provide a more realistic Andean topography, or a higher-resolution model (ECHAM-4 T106) leads to an improved δ18O distribution over the tropical Americas with an altitude effect comparable to observations. Water vapor transport and gradual rain out and increasingly depleted composition of water vapor along its trajectory are correctly simulated in both models, although the ECHAM model appears to underestimate the continentality effect over the Amazon basin. A significant dependence of δ18O on the precipitation amount is apparent in both models, in accordance with observations, while the influence of temperature seems to be less significant in most regions and is accurately reproduced by the ECHAM model only. Over most regions, however, the δ18O signal in precipitation is influenced by a combination of factors, such as precipitation amount, temperature, moisture source variability, and atmospheric circulation changes. Over parts of the tropical Americas, the δ18O signal is therefore also significantly correlated with ENSO because ENSO is an integrator of many factors affecting the δ18O composition of precipitation.