Stable water isotopes have been added to the full hydrological cycle of the Hadley Centre Climate model (HadCM3) coupled atmosphere-ocean GCM. Simulations of δ ¹⁸O in precipitation and at the ocean surface compare well with observations for the present-day climate. The model has been used to investigate the isotopic anomalies associated with ENSO; it is found that the anomalous δ ¹⁸O in precipitation is correlated with the anomalous precipitation amount in accordance with the “amount effect.” The El Niño δ ¹⁸O anomaly at the ocean surface is largest in coastal regions because of the mixing of ocean water and the more depleted runoff from the land surface. Coral δ ¹⁸O anomalies were estimated, using an established empirical relationship, and generally reflect ocean surface δ ¹⁸O anomalies in coastal regions and sea surface temperatures away from the coast. The spatial relationship between tropical precipitation and δ ¹⁸O was investigated for the El Niño anomaly simulated by HadCM3. Weighting the El Niño precipitation anomaly by the precipitation amount at each grid box gave a large increase in the spatial correlation between tropical precipitation and δ ¹⁸O. This improvement was most apparent over land points and between 10 and 20° of latitude.