Molecular and isotopic compositions of nine gas samples from seven fields in the Thrace basin (northwest Turkey) are determined in order to define the characteristics of the respective possible source rocks. According to previous organic geochemical studies, the Hamitabat, Ceylan and Mezardere formations are considered as potential source rocks. A first evaluation of the delta(13)C values of methane, ethane, propane and the deltaD isotope value of methane indicated in general a thermogenic origin. However, a further evaluation, which considers the isotopic composition of delta(13)C from methane, ethane and propane vs. 1/n, where n is the respective number of carbon atoms in each gas molecule, showed mixing of gases of different origin in most of the analysed gas samples. Consequently, three gas groups are distinguished: group A-a thermogenic gas from a single source rock, group B-a mixture of thermogenic and microbial gases, group C-a mixture of thermogenic gases at least from two different source rocks.