A method is described for ultrasonic vacuum extraction of gases from groundwater samples. Ultrasonic vacuum extraction provides near-quantitative gas yields within 30 min, and allows on-line preparation of carbon dioxide, nitrogen and oxygen for isotopic analyses. This method eliminates uncertainties involved in correcting headspace gas analyses for solubilities and liquid-vapor isotopic fractionation. In addition to developmental data, some field results are given to demonstrate the applicability of the method to geochemical studies.