The present investigation focused on the origin of CO2 and CH4 in the soil gas and two gas vents present in the Fucino plain, a tectonically active intramontane depression of central Italy. The soil gases have concentrations of CO2 ranging from 1 to 10% and CH4 in the order of a few parts per million. Using carbon isotopes, it was possible to identify two different sources of CO2 in the soil gas: biogenic produced in the fluvio-lacustrine sediments of the plain and CO2 produced from CH4 oxidation in soil . The two gas vents yield CO2 contents of 2.63 and 15.33% and CH4 of 94.7 and 8.5%. In the CH4-dominated gas vent, the carbon isotopes of methane point to a gas of microbial origin. The other gas vent, with a of −52.8‰, may be affected by a microbial oxidation process.