Noble gas analysis permits the resolution of the lithospheric mantle contribution from both the crustal and groundwater-derived inputs within natural CO2 reservoirs (Ballentine et al., 1992). Determination of these crustal and groundwater sources provides new information about the role of the groundwater and the regional focussing and transport of magmatic CO2 and other inorganic natural gases in the subsurface. Hence, the data presented here from SW US natural CO2 fields provides an important analogue relevant to understanding the integrity of programs investigating the geological option for sequestration of anthropogenic CO2. We have extensively studied CO2 well gases from three systems to the east of the Colorado Plateau uplift province, and two systems from within the uplift area, namely Bravo Dome (Harding Co, NM) (Ballentine et al., 1992), Sheep Mountain (Huerfano Co, CO), McCallum Dome, (Jackson Co, CO), St. Johns Dome (Apache Co, AZ) and McElmo Dome (Montezuma Co, CO). Our study is the first to determine a magmatic origin for the CO2 in these fields, by comparing ?13C(CO2) and CO2/3He ranges observed within the gas fields to those of MORB. Additionally, we have developed an innovative CO2 redissolution model that can account for the clear reduction of the original magmatic CO2/3He ratio and the fractionation of ?13C(CO2) isotopes observed within all of the fields. This enables the processes responsible for lowering the CO2/3He ratio, namely CO2 dissolution into the groundwater and CO2 precipitation as CaCO3, to be distinguished and quantified. Importantly, this model highlights that in several of the reservoirs, precipitation of CaCO3 is not a significant process in reducing the original CO2/3He ratio compared to dissolution of CO2 into the groundwater.