Seven coreholes, 63 to 533 m deep, were drilled on Ascension Island to determine the shallow thermal structure in preparation for siting a deep geothermal exploration well. These holes were used to evaluate specific geothermal exploration targets defined by previously completed geological mapping, aeromagnetic and electrical resistivity surveys. The highest temperature measured was 54.4 degrees C at 203 m below sea-level. Measured temperature gradients reached up to 72 degrees C/km; heat flow, up to 124 mW/m(2). The higher heat flow values are associated with the two silicic volcanic centers on the island. The water table in all wells is at approximately sea-level. Groundwater samples from different depths in corehole LDTGH, located in the central part of the island, show a zone of brackish water overlying water with the salinity of seawater. Both waters retain the cation signature of seawater, but show elevated silica concentrations. Mixing calculations and chemical geothermometry indicate temperatures as high as 143 degrees C.