The isotopic composition of carbon monoxide (13C,14C,18O) and methane (13C,D) was measured on air samples collected between Vladivostok and Moscow using the Trans-Siberian railroad during August 1996. Apart from short term fluctuations in the direct vicinity of sources, continuous measurements of CO and CH4 showed sustained, elevated mixing ratios over several hundreds of kilometers indicating the large scale influencing of traversed air masses by significant sources. Persistent, enhanced CH4 levels were found over the west Siberian lowlands concurrent with significantly depleted Δ13C and ΔD values. The derived isotopic signature of the CH4 source (Δ13C=-62.5¿4.7? V-PDB; ΔD=-311¿14? V-SMOW) clearly indicates the dominance of biogenic CH4, with the west Siberian wetlands being the most likely candidate. A second major feature in the data set is the enormous enhancement of CO (up to 1500 nmol/mol) east of Chita, extending over a 2000 km section along the river Amur. The 14CO measurements and back trajectory analyses identify biomass burning as the origin of the highly elevated CO. This is further supported by the Δ18O(CO) and the Δ13C/ΔD signature of the accompanying moderate CH4 enhancement. ¿ 1998 American Geophysical Union<