Thermal waters in Matupi Harbour and Sulphur Creek, Rabaul caldera have D/H and O18/O16 ratios that are indicative of a mixed source. They are the result of mixing of local meteoric waters with hot water of marine origin. The stable isotope data are grouped into distinct areas close to the meteoric water line. They suggest that the thermal systems away from the shoreline are dominated by meteoric water and that warmed sea water only enters the springs at the shoreline. Low temperature (100°C) fumarolic exhalations from Tavurvur and Rabalankaia volcanoes consist largely of recycled meteoric water. These conclusions conflict in part with those drawn from anion ratio and trace metal contents which were inferred by previous authors to be consistent with an hypothesis of modified sea water origin. We suggest that the chemistry of these acid, mineralised geothermal waters is a reflection of their later, near surface, history and does not necessarily give a correct picture of their ultimate origin. The enhanced Fe, Mn, and Zn values of the Matupi springs are a function of the leaching potential of geothermal fluids at elevated temperatures, and of the chemistry of the porous and chemically reactive rocks through which they pass.