Central Italy is characterized by an anomalous flux of deeply derived CO2. In the western Tyrrhenian sector of central Italy, CO2 degassing occurs mainly from focused emissions (vents and strong diffuse degassing) and thermal springs, whereas in the eastern Apennine area, deep CO2 is dissolved in cold groundwaters of regional aquifers hosted by Mesozoic carbonate-evaporite formations. Influx of deep CO2 into 12 carbonate aquifers (12,500 km2) of the central Apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data. Mass balance calculations estimate that 6.5¿1010 mol yr-1 of inorganic carbon are dissolved in the studied aquifers. Approximately 23% of this amount derives from biological sources active during the infiltration of the recharge waters, 36% comes from carbonate dissolution, while 41% is representative of deep carbon sources characterized by a common isotopic signature (Δ13C≅-3%). The calcualted deep CO2 influx rate ranges from 105 to 107 mol yr-1 km-2, increasing regionally from east to west in the study area. ¿ 2000 American Geophysical Union