Gas exhalations in the area around Bongwan near Port Shepstone, KwaZulu-Natal, consist of more than 97% carbon dioxide. Emissions of CO 2 gas of up to 46 kg/hour are associated with the north-south-trending Bongwan gas fault which extends for at least 80 km. The CO 2 has delta 13 C values ranging from -0.6 to +0.9 per mil (relative to PDB) and delta 18 O values ranging from 35.3 to 45.1 per mil (relative to SMOW). On the basis of the carbon isotope ratios, the origin of the CO 2 is most likely to be the reaction of acid groundwater with carbonate rocks at depth. The high delta 18 O values are consistent with such a model, with reaction taking place at temperatures less than about 50 degrees C. However, interpretations based on oxygen isotope data must be treated with caution because high delta 18 O values would also have resulted from equilibration of the CO 2 with groundwater on the way to the surface.