Air samples collected using aircraft during the Siberian Terrestrial Ecosystem-Atmosphere-Cryosphere Experiments (STEACE) in the summer of 1993 and 1994 were analyzed for the carbon isotopic ratio, δ13C, of atmospheric CH4 as well as for the CH4 concentration. The CH4 concentrations and δ13C values observed in the lower troposphere over wetlands in the West Siberian Lowland varied considerably, showing a clear negative correlation between the two components. From the relationships between measured values of the CH4 concentration and δ13C, values of δ13C of CH4 released from wetlands into the atmosphere were estimated to be −75 to −67‰. The results observed over oil wells and pipelines showed isotopic evidence for leakage of natural gas. Mean values of δ13C measured in the middle and upper troposphere over Siberia in the summer season were −47.9±0.3 and −47.8±0.2‰ for 1993 and 1994, respectively, which are quite similar to each other.