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, &dgr;13C, of atmospheric CH4 as well as for the CH4 concentration. The CH4 concentrations and &dgr;13C 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 &dgr;13C, values of &dgr;13C 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 &dgr;13C 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. ¿ American Geophysical Union 1996