In Tyrrhenian Central Italy large amounts of CO2 are produced at depth mainly by metamorphism of marine carbonate rocks. During their ascent, the gases are trapped in deep structures, made up of Mesozoic permeable limestones covered by impermeable terrains, which become sources of a high CO, flux toward the surface. The anomalous CO2 concentrations are delectable in groundwaters. The general map of the groundwater PCO2 values shows a decrease in the CO2 production moving from the western geothermal areas of Tuscany and Latium to the eastern sector, which is characterized by a normal heat flow. The PCO2 distribution suggests that the NW-SE extensional faults, which bound Plio-Pleistocene grabens, provide the easiest routes for the gas ascent. The geological, geophysical and geothermal deep drilling data suggest that the CO2 anomalies found in the western sector correspond to deep permeable structures of possible geothermal interest.