We examine the high-resolution (∼32 samples/year) carbon and oxygen isotopic composition (δ 13C coral and δ 18O coral) in a coral core ( Porites lobata) from Double Reef, Guam over the years 1980–2000. The δ 13C coral shows clear seasonal variations with mean seasonal amplitude of 1.89‰, which roughly corresponds with seasonal variations in solar radiation. The seasonal amplitude of δ 18O coral variations are small (0.23–0.57‰), but they are significantly correlated with sea surface temperature (SST) and salinity (SSS). The δ 18O coral and SST are more strongly correlated during El Niño/Southern Oscillation (ENSO) warm phases ( r=−0.81, p<0.01) than during non-ENSO phases ( r=−0.65, p<0.01) and ENSO cool phases ( r=−0.48, p<0.01). These different relationships are due to differences in winter SST and in seawater δ 18O (δ 18O sw) during ENSO warm phases (<27 °C and higher values of δ 18O sw) compared with cool phases (>28 °C and lower values of δ 18O sw) at Guam. These differences in oceanic parameters result from movements of the Western Pacific Warm Pool (WPWP) during the different phases of ENSO. Anomalies in δ 18O sw, inferred from the δ 18O coral and instrumental SST, are consistent with SSS anomalies for the years 1980–2000. These δ 18O sw anomalies may reflect changes in SSS and evaporation–precipitation due to movements of the WPWP. This detailed analysis of a coral from Guam suggests that it may contain an excellent archive of past ENSO events.