A soil gas and CO2 efflux survey was carried out at Hakkoda, north Japan, to investigate the spatial distribution and origin of the gas which caused the accidental death of three Japanese soldiers on 12 July 1997. In the study area, no fumarolic and/or thermal activity is observed. Chemical composition of the gas from areas with high CO2 effluxes (>5,000 g m-2 d-1) and high CO2 contents (>15 vol %) indicate a strong deep sourced contribution for CO2, whereas air and/or air saturated water is suggested for the rest of the discharged gases, which are mainly released from a shallow aquifer. Isotopic analysis of soil gas and hot spring discharges suggests a combination of magmatic degassing and thermal decarbonation, with magmatic degassing being the main contributior to the high CO2 emission. Approximately 74 t d-1 of deep-seated (magmatic plus thermal decarbonation origin) CO2 were estimated to contribute to the total output of CO2 from this area.