The occurrence of carbon dioxide (CO2) in Central Anatolia has been well-known for 50 years. Four gas producing fields (Bozkar, Guneygaz, Karbogaz, and Terra) are located in the region. The aim of this study was to define the composition of these gas occurrences and to determine the gas origin. For this purpose, the molecular and isotopic composition of gas samples from production wells were investigated and interpreted. Gases from the region are rich in CO2 (>80%). Carbon dioxide concentrations were found to vary from 80.7 to 94.3%. The total amount of methane components (CH4+) was less than 0.1%. The isotopic composition of CO2, with values between -1.76 and 1.02 parts per thousand, indicated abiogenic origin from limestone. The generation of CO2 could largely result from the decarbonation of limestone. Helium isotopic values indicated that a minor amount of CO2 originated from the mantle. delta C-13 values for methane from the Karbogaz Field suggested that the origin of the Aksaray Field methane is thermogenic.